lib: utils: Extend fdt_get_node_addr_size() for multiple register sets

We add "index" parameter to fdt_get_node_addr_size() API so that
calling function can specify index of desired register set. This
will allow fdt_get_node_addr_size() to handle DT nodes with
multiple register sets.

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