irqchip: Add driver for the RPMI system MSI service group

The RPMI specification defines a system MSI service group which
allows application processors to receive MSIs upon system events
such as graceful shutdown/reboot request, CPU hotplug event, memory
hotplug event, etc.

Add an irqchip driver for the RISC-V RPMI system MSI service group
to directly receive system MSIs in Linux kernel.

Reviewed-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Anup Patel <apatel@ventanamicro.com>
Link: https://lore.kernel.org/r/20250818040920.272664-14-apatel@ventanamicro.com


Signed-off-by: Paul Walmsley <pjw@kernel.org>