irqchip/riscv-imsic: Add a CPU pm notifier to restore the IMSIC on exit

irqchip/riscv-imsic: Add a CPU pm notifier to restore the IMSIC on exit

The IMSIC might be reset when the system enters a low power state, but on
exit nothing restores the registers, which prevents interrupt delivery.

Solve this by registering a CPU power management notifier, which restores
the IMSIC on exit.

Signed-off-by: Nick Hu <nick.hu@sifive.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Yong-Xuan Wang <yongxuan.wang@sifive.com>
Reviewed-by: Cyan Yang <cyan.yang@sifive.com>
Reviewed-by: Anup Patel <anup@brainfault.org>
Reviewed-by: Nutty Liu <liujingqi@lanxincomputing.com>
Link: https://patch.msgid.link/20251202-preserve-aplic-imsic-v3-1-1844fbf1fe92@sifive.com