regulator: act8945a-regulator: Implement PM functionalities

The regulator supports a dedicated suspend mode.
Implement the appropriate ->set_suspend_xx() hooks, add support for
->set_mode(), and provide basic PM ops functionalities to setup the
regulator in a suspend state when the system is entering suspend.

Signed-off-by: Boris Brezillon <boris.brezillon@bootlin.com>
[claudiu.beznea@microchip.com: remove shutdown function, use dev_pm_ops,
 fix checkpatch warning, adapt commit message, add LDO modes support,
 move modes constants to active-semi,8945a-regulator.h, remove rdevs from
 struct act8945a_pmic, add op_mode to act8945a_pmic]
Signed-off-by: Claudiu Beznea <claudiu.beznea@microchip.com>
Signed-off-by: Mark Brown <broonie@kernel.org>