genirq: Add mechanism to multiplex a single HW IPI

All RISC-V platforms have a single HW IPI provided by the INTC local
interrupt controller. The HW method to trigger INTC IPI can be through
external irqchip (e.g. RISC-V AIA), through platform specific device
(e.g. SiFive CLINT timer), or through firmware (e.g. SBI IPI call).

To support multiple IPIs on RISC-V, add a generic IPI multiplexing
mechanism which help us create multiple virtual IPIs using a single
HW IPI. This generic IPI multiplexing is inspired by the Apple AIC
irqchip driver and it is shared by various RISC-V irqchip drivers.

Signed-off-by: Anup Patel <apatel@ventanamicro.com>
Reviewed-by: Hector Martin <marcan@marcan.st>
Tested-by: Hector Martin <marcan@marcan.st>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20230103141221.772261-4-apatel@ventanamicro.com