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>

common/image-fdt.c

@@ -454,6 +454,11 @@ __weak int ft_verify_fdt(void *fdt)
 	return 1;
 }
 
+__weak int arch_fixup_fdt(void *blob)
+{
+	return 0;
+}
+
 int image_setup_libfdt(bootm_headers_t *images, void *blob,
 		       int of_size, struct lmb *lmb)
 {