arm64: Specify SMC as the PSCI conduits for realms

This lets the VM use the RMM implementation for PSCI.

Signed-off-by: Christoffer Dall <christoffer.dall@arm.com>
Signed-off-by: Alexandru Elisei <alexandru.elisei@arm.com>
Signed-off-by: Suzuki K Poulose <suzuki.poulose@arm.com>