binman: Support default alignment for sections

Sometimes it is useful to specify the default alignment for all entries
in a section, such as when word-alignment is necessary, for example. It
is tedious and error-prone to specify this individually for each section.

Add a property to control this for a section.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/tools/binman/binman.rst b/tools/binman/binman.rst
index 28cb2e7..1aa2459 100644
--- a/tools/binman/binman.rst
+++ b/tools/binman/binman.rst
@@ -562,6 +562,13 @@
     'end-at-4gb' property is not applicable where CONFIG_SYS_TEXT_BASE +
     Image size != 4gb.
 
+align-default:
+    Specifies the default alignment for entries in this section, if they do
+    not specify an alignment. Note that this only applies to top-level entries
+    in the section (direct subentries), not any subentries of those entries.
+    This means that each section must specify its own default alignment, if
+    required.
+
 Examples of the above options can be found in the tests. See the
 tools/binman/test directory.