regulator: act8945a-regulator: unlock expert registers

Unlock expert registers for act8945a.
This is based on orginal work of Boris Brezillon at [1].

[1] https://www.spinics.net/lists/kernel/msg2942960.html



Signed-off-by: Claudiu Beznea <claudiu.beznea@microchip.com>
Signed-off-by: Mark Brown <broonie@kernel.org>