arm64: RME: Add SMC definitions for calling the RMM

The RMM (Realm Management Monitor) provides functionality that can be
accessed by SMC calls from the host.

The SMC definitions are based on DEN0137[1] version A-bet0.

[1] https://developer.arm.com/documentation/den0137/latest



Signed-off-by: Steven Price <steven.price@arm.com>