drivers: intc: plic: remove incorrect arch_proc_id() usage

The `arch_proc_id()` returns the hartid of a CPU, which may not start
from zero. The way that it's used as an index to access `save_irq[]`
array is wrong, use `arch_curr_cpu()->id` instead.

Signed-off-by: Yong Cong Sin <ycsin@meta.com>
Signed-off-by: Yong Cong Sin <yongcong.sin@gmail.com>