dt-bindings: Add RPMI system MSI interrupt controller bindings

dt-bindings: Add RPMI system MSI interrupt controller bindings

Add device tree bindings for the RPMI system MSI service group
based interrupt controller for the supervisor software.

The RPMI system MSI service group is defined by the RISC-V
platform management interface (RPMI) specification.

Reviewed-by: Atish Patra <atishp@rivosinc.com>
Reviewed-by: Conor Dooley <conor.dooley@microchip.com>
Signed-off-by: Anup Patel <apatel@ventanamicro.com>
Acked-by: Jassi Brar <jassisinghbrar@gmail.com>
Link: https://lore.kernel.org/r/20250818040920.272664-13-apatel@ventanamicro.com
Signed-off-by: Paul Walmsley <pjw@kernel.org>