dtoc: Correct quotes in fdt_util

The style is to use single quotes for strings where possible. Adjust this
function.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/tools/dtoc/fdt_util.py b/tools/dtoc/fdt_util.py
index 3e25a8b..32f41d7 100644
--- a/tools/dtoc/fdt_util.py
+++ b/tools/dtoc/fdt_util.py
@@ -22,7 +22,7 @@
     Return:
         A native-endian integer value
     """
-    return struct.unpack(">I", val)[0]
+    return struct.unpack('>I', val)[0]
 
 def EnsureCompiled(fname):
     """Compile an fdt .dts source file into a .dtb binary blob if needed.