dt-bindings: clock: Add RPMI clock service message proxy bindings

dt-bindings: clock: Add RPMI clock service message proxy bindings

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

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

Reviewed-by: Conor Dooley <conor.dooley@microchip.com>
Signed-off-by: Anup Patel <apatel@ventanamicro.com>
Acked-by: Stephen Boyd <sboyd@kernel.org>
Link: https://lore.kernel.org/r/20250818040920.272664-9-apatel@ventanamicro.com
Signed-off-by: Paul Walmsley <pjw@kernel.org>