arm: realm: Realm initialisation

During the boot, run a check for the presence of RMM. If we are Realm,
detect the Realm configuration using RSI and initialise the key parameters.

Also expose a helper to indicate if this is running inside a Realm

Co-developed-by: Suzuki K Poulose <suzuki.poulose@arm.com>
Signed-off-by: Suzuki K Poulose <suzuki.poulose@arm.com>
Signed-off-by: Joey Gouly <joey.gouly@arm.com>