arm64: kernel: enable PSCI cpu operations on UP systems

arm64: kernel: enable PSCI cpu operations on UP systems

PSCI CPU operations have to be enabled on UP kernels so that calls
like eg cpu_suspend can be made functional on UP too.

This patch reworks the PSCI CPU operations so that they can be
enabled on UP systems.

Acked-by: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>
(cherry picked from commit 756854d9b99a735f86bc3b86df5c19be12e8746e)
Signed-off-by: Mark Brown <broonie@kernel.org>