irqchip/sifive-plic: Mark two global variables __ro_after_init

All of these two are never modified after init, so they can be
 __ro_after_init.

Signed-off-by: Jisheng Zhang <jszhang@kernel.org>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20210330020911.26423e9e@xhacker