It is possible that a PLIC IRQ is claimed but not completed at warm
reset. This occurs at least if there is a fault in the middle of irq
handler execution.
To recover from such situation, we can complete all IRQ:s in PLIC;
this completes any already claimed IRQ, but has no effect on IRQs
which are not claimed or not enabled.
Signed-off-by: Jukka Laitinen <jukkax@ssrc.tii.ae>