RISC-V: Add defines for the SBI message proxy extension

RISC-V: Add defines for the SBI message proxy extension

Add defines for the new SBI message proxy extension which is part
of the SBI v3.0 specification.

Reviewed-by: Atish Patra <atishp@rivosinc.com>
Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Co-developed-by: Rahul Pathak <rpathak@ventanamicro.com>
Signed-off-by: Rahul Pathak <rpathak@ventanamicro.com>
Signed-off-by: Anup Patel <apatel@ventanamicro.com>
Link: https://lore.kernel.org/r/20250818040920.272664-4-apatel@ventanamicro.com
Signed-off-by: Paul Walmsley <pjw@kernel.org>