mfd: axp20x: Add support for RSB based AXP223 PMIC

The AXP223 is a new PMIC commonly paired with Allwinner A23/A33 SoCs.
It is functionally identical to AXP221; only the regulator default
voltage/status and the external host interface are different.

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