irqchip/irq-sifive-plic: Add syscore callbacks for hibernation

The priority and enable registers of plic will be reset
during hibernation power cycle in poweroff mode,
add the syscore callbacks to save/restore those registers.

Signed-off-by: Mason Huo <mason.huo@starfivetech.com>
Reviewed-by: Ley Foon Tan <leyfoon.tan@starfivetech.com>
Reviewed-by: Sia Jee Heng <jeeheng.sia@starfivetech.com>
Reported-by: Dan Carpenter <error27@gmail.com>
Link: https://lore.kernel.org/r/202302140709.CdkxgtPi-lkp@intel.com/


Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20230404032908.89638-1-mason.huo@starfivetech.com