fdt: Implement weak arch_fixup_fdt()

Only ARM and in some configs MIPS really implement arch_fixup_fdt().
Others just use the same boilerplate which is not good by itself,
but what's worse if we try to build with disabled CONFIG_CMD_BOOTM
and enabled CONFIG_OF_LIBFDT we'll hit an unknown symbol which was
apparently implemented in arch/xxx/lib/bootm.c.

Now with weak arch_fixup_fdt() right in image-fdt.c where it is
used we get both items highlighted above fixed.

Signed-off-by: Alexey Brodkin <abrodkin@synopsys.com>
Cc: Daniel Schwierzeck <daniel.schwierzeck@gmail.com>
Cc: Simon Glass <sjg@chromium.org>
Cc: York Sun <york.sun@nxp.com>
Cc: Stefan Roese <sr@denx.de>
Reviewed-by: Tom Rini <trini@konsulko.com>
Reviewed-by: Daniel Schwierzeck <daniel.schwierzeck@gmail.com>

arch/x86/lib/bootm.c

@@ -28,11 +28,6 @@ DECLARE_GLOBAL_DATA_PTR;
 
 #define COMMAND_LINE_OFFSET 0x9000
 
-int arch_fixup_fdt(void *blob)
-{
-	return 0;
-}
-
 __weak void board_quiesce_devices(void)
 {
 }