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>