dtoc: Support processing the root node

The device for the root node is normally bound by driver model on init.
With devices being instantiated at build time, we must handle the root
device also.

Add support for processing the root node, which may not have a compatible
string.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/tools/dtoc/src_scan.py b/tools/dtoc/src_scan.py
index 8619206..114212c 100644
--- a/tools/dtoc/src_scan.py
+++ b/tools/dtoc/src_scan.py
@@ -33,6 +33,8 @@
     new = new.replace('-', '_')
     new = new.replace(',', '_')
     new = new.replace('.', '_')
+    if new == '/':
+        return 'root'
     return new
 
 def get_compat_name(node):
@@ -250,7 +252,10 @@
                 In case of no match found, the return will be the same as
                 get_compat_name()
         """
-        compat_list_c = get_compat_name(node)
+        if not node.parent:
+            compat_list_c = ['root_driver']
+        else:
+            compat_list_c = get_compat_name(node)
 
         for compat_c in compat_list_c:
             if not compat_c in self._drivers.keys():
@@ -509,21 +514,23 @@
                 elif m_hdr:
                     driver.headers.append(m_hdr.group(1))
                 elif '};' in line:
-                    if driver.uclass_id and compat:
-                        if compat not in of_match:
-                            raise ValueError(
-                                "%s: Unknown compatible var '%s' (found: %s)" %
-                                (fname, compat, ','.join(of_match.keys())))
-                        driver.compat = of_match[compat]
+                    is_root = driver.name == 'root_driver'
+                    if driver.uclass_id and (compat or is_root):
+                        if not is_root:
+                            if compat not in of_match:
+                                raise ValueError(
+                                    "%s: Unknown compatible var '%s' (found: %s)" %
+                                    (fname, compat, ','.join(of_match.keys())))
+                            driver.compat = of_match[compat]
 
-                        # This needs to be deterministic, since a driver may
-                        # have multiple compatible strings pointing to it.
-                        # We record the one earliest in the alphabet so it
-                        # will produce the same result on all machines.
-                        for compat_id in of_match[compat]:
-                            old = self._compat_to_driver.get(compat_id)
-                            if not old or driver.name < old.name:
-                                self._compat_to_driver[compat_id] = driver
+                            # This needs to be deterministic, since a driver may
+                            # have multiple compatible strings pointing to it.
+                            # We record the one earliest in the alphabet so it
+                            # will produce the same result on all machines.
+                            for compat_id in of_match[compat]:
+                                old = self._compat_to_driver.get(compat_id)
+                                if not old or driver.name < old.name:
+                                    self._compat_to_driver[compat_id] = driver
                         drivers[driver.name] = driver
                     else:
                         # The driver does not have a uclass or compat string.