regulator: vexpress: Constify regulator_ops

vexpress_regulator_ops_ro and vexpress_regulator_ops are never changed,
make them const.

Signed-off-by: Axel Lin <axel.lin@ingics.com>
Acked-by: Sudeep Holla <sudeep.holla@arm.com>
Signed-off-by: Mark Brown <broonie@kernel.org>