mfd: axp20x: Add support for AXP806 PMIC

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

This PMIC has a new register layout, and supports some functions not
seen in other X-Powers PMICs, such as master-slave mode, or having
multiple AXP806 PMICs on the same bus with address space extension,
or supporting both I2C and RSB mode. I2C has not been tested.

This patch adds support for the interrupts of the PMIC. A regulator
sub-device is enabled, but actual regulator support will come in a
later patch.

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