irqchip/sifive-plic: Support wake IRQs

The PLIC does not define any special method for marking interrupts as
wakeup-capable, so it should have the IRQCHIP_SKIP_SET_WAKE flag set.

Signed-off-by: Samuel Holland <samuel@sholland.org>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20221126194805.19431-1-samuel@sholland.org