psci: Add accessor for psci_0_1_function_ids

Make it possible to retrieve a copy of the psci_0_1_function_ids struct.
This is useful for KVM if it is configured to intercept host's PSCI SMCs.

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/20201202184122.26046-7-dbrazdil@google.com