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

We 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>
Reviewed-by: Conor Dooley <conor.dooley@microchip.com>
Acked-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>