binman: Correct the comment for fdtgrep

This returns stdout, not a CommandResult so update the comment.

Signed-off-by: Simon Glass <sjg@chromium.org>

tools/binman/btool/fdtgrep.py

@@ -74,8 +74,7 @@ class Bintoolfdtgrep(bintool.Bintool):
                 (with only neceesary nodes and properties)
 
         Returns:
-            CommandResult: Resulting output from the bintool, or None if the
-                tool is not present
+            str or bytes: Resulting stdout from the bintool
         """
         if phase == 'tpl':
             tag = 'bootph-pre-sram'

tools/binman/etype/fit.py

@@ -550,6 +550,9 @@ class Entry_fit(Entry_section):
             phase (str): Phase to generate for ('tpl', 'vpl', 'spl')
             outfile (str): Output filename to write the grepped FDT contents to
                 (with only neceesary nodes and properties)
+
+        Returns:
+            str or bytes: Resulting stdout from fdtgrep
         """
         return self.fdtgrep.create_for_phase(infile, phase, outfile,
                                              self._remove_props)