mfd: axp20x: Add support for AXP813 PMIC

The X-Powers AXP813 PMIC is normally used with Allwinner's A83T SoC.
It has the same range of functions as other X-Powers PMICs, such as
DC-DC buck converter and linear regulator outputs, AC-IN and VBUS
power supplies, power button trigger, GPIOs, ADCs, and a battery
charger.

Note that the IRQ table given in the datasheet is incorrect: in IRQ
enable/status registers 1, there are separate IRQs for ACIN and VBUS,
instead of bits [7:5] being the same as bits [4:2]. So it shares the
same IRQs as the AXP803, rather than the AXP288.

This patch adds basic mfd support for it, with only the power button
enabled.

Signed-off-by: Chen-Yu Tsai <wens@csie.org>
Acked-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>