drivers: intc: plic: fix IRQ on every hart regardless of mapping

Allow IRQs to work on every hart regardless of the mapping
of the contexts.

Add a test to validate the hart-context mapping.

Signed-off-by: Yong Cong Sin <ycsin@meta.com>
Signed-off-by: Yong Cong Sin <yongcong.sin@gmail.com>
(cherry picked from commit 475ff826)