irqchip/sifive-plic: Support irq domain hierarchy

Add support for hierarchical irq domains. This is needed as
pre-requisite for gpio-sifive driver.

Signed-off-by: Yash Shah <yash.shah@sifive.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/1575976274-13487-4-git-send-email-yash.shah@sifive.com