ppc4xx/fdt/flash: Fix bug in fdt_fixup_nor_flash_node()

This patch fixes a bug in fdt_fixup_nor_flash_node() when the reg
property has multiple reg tuples, like:

	reg = <0 0x00000000 0x04000000
	       0 0x04000000 0x04000000>;

In this case this function did not update the reg property correctly.

Signed-off-by: Stefan Roese <sr@denx.de>