dt-bindings: interrupt-controller: Add RISC-V incoming MSI controller

Add DT bindings document for the RISC-V incoming MSI controller (IMSIC)
defined by the RISC-V advanced interrupt architecture (AIA) specification.

Signed-off-by: Anup Patel <apatel@ventanamicro.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Tested-by: Björn Töpel <bjorn@rivosinc.com>
Reviewed-by: Conor Dooley <conor.dooley@microchip.com>
Reviewed-by: Björn Töpel <bjorn@rivosinc.com>
Acked-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Link: https://lore.kernel.org/r/20240307140307.646078-2-apatel@ventanamicro.com