arm64: Add skeleton implementation for PSCI

Add an extremely barebones implementation for handling PSCI, where the
only supported call is PSCI_VERSION.

Reviewed-by: Marc Zyngier <maz@kernel.org>
Signed-off-by: Oliver Upton <oliver.upton@linux.dev>
Signed-off-by: Suzuki K Poulose <suzuki.poulose@arm.com>
Signed-off-by: Will Deacon <will@kernel.org>
6 files changed