arm64: rsi: Add RSI definitions

The RMM (Realm Management Monitor) provides functionality that can be
accessed by a realm guest through SMC (Realm Services Interface) calls.

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

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



Signed-off-by: Suzuki K Poulose <suzuki.poulose@arm.com>
Signed-off-by: Steven Price <steven.price@arm.com>