mfd: axp20x: Support AXP803 variant

AXP803 is a new PMIC chip produced by X-Powers, usually paired with A64
via RSB bus. The PMIC itself is like AXP288, but with RSB support and
dedicated VBUS and ACIN.

Add support for it in the axp20x mfd driver.

Currently only power key function is supported.

Signed-off-by: Icenowy Zheng <icenowy@aosc.io>
Signed-off-by: Lee Jones <lee.jones@linaro.org>