fdtdec: Add fdt_{addr, size}_unpack() helpers

These helpers can be used to unpack variables of type fdt_addr_t and
fdt_size_t into a pair of 32-bit variables. This is useful in cases
where such variables need to be written to properties (such as "reg")
of a device tree node where they need to be split into cells.

Signed-off-by: Thierry Reding <treding@nvidia.com>