regulator: act8865: support regulator-pull-down property

AC8865 has internal 1.5k pull-down resistor that can be enabled when LDO
is shut down.

Signed-off-by: Michał Mirosław <mirq-linux@rere.qmqm.pl>
Link: https://lore.kernel.org/r/d7338f0dfcac63eb958a6b5e42e2d540b3d3f54a.1563898936.git.mirq-linux@rere.qmqm.pl
Signed-off-by: Mark Brown <broonie@kernel.org>