lib: utils: Add fdt_parse_max_hart_id() API

We add fdt_parse_max_hart_id() API which return max HART id based
on CPU DT nodes. This will be used by generic FDT based drivers in
subsequent patches.

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