riscv: andes_plic: init plic by scanning each cpu node

riscv: andes_plic: init plic by scanning each cpu node

Initialize plic driver by ofnode_for_each_subnode() instead
of cpu_get_count().

This way can support to skip some harts which maybe marked as
unavailable, but the cpu node exists indeed.

Signed-off-by: Rick Chen <rick@andestech.com>
Cc: KC Lin <kclin@andestech.com>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>