irqchip: Add RISC-V incoming MSI controller driver

The RISC-V advanced interrupt architecture (AIA) specification defines
a new MSI controller for managing MSIs and IPIs on a RISC-V platform.

This new MSI controller is referred to as incoming message signalled
interrupt controller (IMSIC) which manages MSI on per-HART (or per-CPU)
basis. (For more details refer https://github.com/riscv/riscv-aia

)

This patch adds an irqchip driver for RISC-V IMSIC which provides
IPIs and platform MSIs to the Linux RISC-V kernel.

Signed-off-by: Anup Patel <apatel@ventanamicro.com>