[LIBS] Make libfdt_get_property() more flexible

Currently, libfdt_get_property() assumes:
address_cells = sizeof(physical_addr_t) / 4
size_cells = sizeof(physical_size_t) / 4

This forces us to set fixed/appropriate values in "#address-cells"
and "#size-cells" attributes of "/" and "/memory" node.

This patch remove the above described limitation and makes
libfdt_get_property() more flexible so that we can have
different values of "#address-cells" and "#size-cells" attributes.

Signed-off-by: Anup Patel <anup@brainfault.org>