common: fdt_support: Check mtdparts cell size

When using fdt_fixup_mtdparts() offset and length cell sizes
are limited to 4 bytes (1 cell). However if the mtd device is
bigger then 4GiB, then #address-cells and #size-cells are
8 bytes (2 cells) [1].

This patch read #size-cells and uses either fdt32_t or
fdt64_t cell size. The default is fdt32_t.

[1] Documentation/devicetree/bindings/mtd/partition.txt

Signed-off-by: Stefan Mavrodiev <stefan@olimex.com>
Reviewed-by: Simon Glass <sjg@chromium.org>