psci: Support psci_ops.get_version for v0.1

KVM's host PSCI SMC filter needs to be aware of the PSCI version of the
system but currently it is impossible to distinguish between v0.1 and
PSCI disabled because both have get_version == NULL.

Populate get_version for v0.1 with a function that returns a constant.

psci_opt.get_version is currently unused so this has no effect on
existing functionality.

Signed-off-by: David Brazdil <dbrazdil@google.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Acked-by: Mark Rutland <mark.rutland@arm.com>
Link: https://lore.kernel.org/r/20201202184122.26046-4-dbrazdil@google.com