mfd: axp20x: Add support for AXP809 PMIC

The X-Powers AXP809 is a new PMIC that is paired with Allwinner's A80
SoC, along with a slave AXP806 PMIC.

This PMIC is quite similar to the earlier AXP223, though the interrupts
and regulator have changed a bit.

This patch adds support for the interrupts and power button of the PMIC.

Signed-off-by: Chen-Yu Tsai <wens@csie.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>