regulator: axp20x: Add support for AXP813 regulators

The AXP813 PMIC has 7 DC-DC buck regulators, 16 LDOs (including the
fixed RTC LDO and 2 GPIO LDOs), and 1 switchable. The drive-vbus
feature is also supported. All the hardware details are very similar
to the AXP803, with the following exceptions:

  - Extra DCDC7 buck regulator, with the same range as DCDC6

  - SWitch now has a separate supply pin, instead of being chained
    internaly from DCDC1

  - RTC LDO output voltage is now 1.8V

  - FLDO3 is an LDO with switchable supplies, but unconfigurable output
    voltage. The voltage is always half that of its supply.

Support for FLDO3 is currently unimplemented, as it requires runtime
switching of its supplies, something the regulator subsystem does not
support. It is not used in either the reference designs nor actually
produced boards available.

Signed-off-by: Chen-Yu Tsai <wens@csie.org>
Tested-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Acked-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Signed-off-by: Mark Brown <broonie@kernel.org>