mfd: axp20x: Allow the AXP803 to be probed by I2C

The AXP803 can be used both using the RSB proprietary bus, or a more
traditional I2C bus.

Let's add that possibility.

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