fdt: Add fdt_fixup_nor_flash_size() to fixup NOR FLASH size in dtb

This function can be used to update the size in the "reg" property
of the NOR FLASH device nodes. This is necessary for boards with
non-fixed NOR FLASH sizes.

Signed-off-by: Stefan Roese <sr@denx.de>
Acked-by: Gerald Van Baren <vanbaren@cideas.com>
Acked-by: Wolfgang Denk <wd@denx.de>