Mark non-executable files non-executable

No need to mark Makefiles, C files or devicetrees
executable.

Change-Id: Ide3a0efc5b14f2cbd7e2a65c541b52491575bb78
Signed-off-by: Patrick Georgi <pgeorgi@google.com>
Reviewed-on: http://review.coreboot.org/7618
Tested-by: build bot (Jenkins)
Reviewed-by: Paul Menzel <paulepanter@users.sourceforge.net>
Reviewed-by: Edward O'Callaghan <eocallaghan@alterapraxis.com>