fdt: Fix regression in fdt_pack_reg()

After commit 933cdbb4: "fdt: Try to use fdt_address_cells()/fdt_size_cells()"
I noticed that allwinner boards would no longer boot.

Switching to fdt_address_cells / fdt_size_cells changes the result from
bytes to 32 bit words, so when we increment pointers into the blob, we must
do so by 32 bit words now.

This commit makes allwinner boards boot again.

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Tested-by: Masahiro Yamada <yamada.m@jp.panasonic.com>
Acked-by: Simon Glass <sjg@chromium.org>
Tested-by: Stefan Roese <sr@denx.de>
Tested-by: Vince Hsu <vinceh@nvidia.com>