arm64: Rename the VHE switch to "finalise_el2"

as we are about to perform a lot more in 'mutate_to_vhe' than
we currently do, this function really becomes the point where
we finalise the basic EL2 configuration.

Reflect this into the code by renaming a bunch of things:
- HVC_VHE_RESTART -> HVC_FINALISE_EL2
- switch_to_vhe --> finalise_el2
- mutate_to_vhe -> __finalise_el2

No functional changes.

Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20220630160500.1536744-2-maz@kernel.org
Signed-off-by: Will Deacon <will@kernel.org>