KVM: arm64: Constify start/end/phys fields of the pgtable walker data

As we are revamping the way the pgtable walker evaluates some of the
data, make it clear that we rely on somew of the fields to be constant
across the lifetime of a walk.

For this, flag the start, end and phys fields of the walk data as
'const', which will generate an error if we were to accidentally
update these fields again.

Reviewed-by: Oliver Upton <oliver.upton@linux.dev>
Signed-off-by: Marc Zyngier <maz@kernel.org>