ARM64: kernel: psci: factor out probe function

PSCI v0.2+ versions provide a specific PSCI call (PSCI_VERSION) to
detect the PSCI version at run-time. Current PSCI v0.2 init code
carries out the version probing in the PSCI 0.2 DT init function,
but the version probing does not depend on DT so it can be factored out
in order to make it available to other boot mechanisms (ie ACPI) to
reuse. The psci_probe() probing function can be easily extended
to add detection and initialization of PSCI functions defined in
PSCI versions >0.2.

Cc: Sudeep Holla <sudeep.holla@arm.com>
Cc: Mark Rutland <mark.rutland@arm.com>
Reviewed-by: Hanjun Guo <hanjun.guo@linaro.org>
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>