lib: utils/fdt: Add fdt_parse_phandle_with_args() API

The libfdt project does not have a generic API to parse phandle
with args from a DT node so we add fdt_parse_phandle_with_args()
for this purpose. This new API will be useful to FDT based drivers.

Signed-off-by: Anup Patel <anup.patel@wdc.com>
Reviewed-by: Atish Patra <atish.patra@wdc.com>