firmware/smccc: Call arch-specific hook on discovering KVM services

arm64 will soon require its own callback to initialise services
that are only available on this architecture. Introduce a hook
that can be overloaded by the architecture.

Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20240830130150.8568-2-will@kernel.org


Signed-off-by: Will Deacon <will@kernel.org>