lib: utils: Make fdt_get_node_addr_size() public function

The fdt_get_node_addr_size() will be useful in FDT based simple
driver frameworks so we make it a public function.

Signed-off-by: Anup Patel <anup.patel@wdc.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>