KVM: arm64: Introduce finalise_el2_state macro

Factor out the first half of the finalise_el2 function into a macro to
allow its reuse from the nVHE PSCI relay code. While at it, make the
register allocation parametric for the check_override macros as they are
now more widely exposed.

No functional changes intended.

Signed-off-by: Quentin Perret <qperret@google.com>
Reviewed-by: Mark Brown <broonie@kernel.org>
Link: https://lore.kernel.org/r/20230201103755.1398086-3-qperret@google.com


Signed-off-by: Oliver Upton <oliver.upton@linux.dev>