dt-bindings: Add RPMI system MSI message proxy bindings

dt-bindings: Add RPMI system MSI message proxy bindings

Add device tree bindings for the RPMI system MSI service group
based message proxy implemented by the SBI implementation (machine
mode firmware or hypervisor).

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/20250818040920.272664-12-apatel@ventanamicro.com
Signed-off-by: Paul Walmsley <pjw@kernel.org>