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 <[email protected]>
Cc: KC Lin <[email protected]>
Reviewed-by: Bin Meng <[email protected]>