lib: utils: Add fdt_parse_plic_node() function

We add fdt_parse_plic_node() function which will allow us to parse
a particular DT node as PLIC node. This will be useful in parsing
the DT node which we have found by matching compatible string.

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