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>