regulator: Add AXP717 boost support

Merge series from Andre Przywara <andre.przywara@arm.com>:

This is remainder of the AXP717 fix series, containing support for the
boost regulator. This is meant to increase the battery voltage to the 5
volts required to provide the USB VBUS power.
It's the usual trinity of DT bindings patch (1/3), the MFD part
describing the PMIC registers (2/3) and the final patch to model the
regulator (3/3).
Compared to v2, this drops the merged patches, and just retains the
boost related parts. It also changes the internal name of the register
to AXP717_MODULE_EN_CONTROL_2, since there is another control register
we will need later for battery support.