mfd: axp20x: Allow the AXP223 to be probed by I2C

The AXP223 can be used both using the RSB proprietary bus, or a more
traditional I2C bus. The RSB is a faster bus and provides more features
(like some integrity checks on the messages), so it's usually preferrable
to use it, but since it's proprietary, when we want to use the PMIC in a
multi-master setup, the i2c might make sense as well.

Let's add that possibility.

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