power: pmic: Add a driver for X-Powers AXP PMICs

power: pmic: Add a driver for X-Powers AXP PMICs

These PMICs provide some combination of battery charger, fuel gauge,
GPIOs, regulators, and VBUS routing. These functions are represented
as child nodes in the device tree. Add the minimal driver needed to
probe these child devices and provide the DM_PMIC ops.

Enable the driver by default for SoCs that normally pair with a PMIC.

Signed-off-by: Samuel Holland <samuel@sholland.org>
Reviewed-by: Andre Przywara <andre.przywara@arm.com>
Signed-off-by: Andre Przywara <andre.przywara@arm.com>