drivers: intc: plic: use per-instance spinlock

Instead of doing an `irq_lock()`, use per-instance spinlock instead.

Refactored out an unlocked version of `local_irq_is_enabled`
from `riscv_plic_irq_is_enabled()` to achieve that.

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