mfd: axp20x: Support AXP806 in I2C mode

The Pine64 H64 board uses an AXP806 PMIC in I2C and self-working mode.
The H64 SoC does not have the usual RSB controller.

This patch adds AXP806 to the list of devices supported in I2C mode.
In theory, all RSB-based PMICs can also be used in I2C mode.

Signed-off-by: Chen-Yu Tsai <wens@csie.org>
Reviewed-by: Icenowy Zheng <icenowy@aosc.io>
Tested-by: Icenowy Zheng <icenowy@aosc.io>
Acked-by: Maxime Ripard <maxime.ripard@bootlin.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>