mfd: axp20x: Add battery power supply cell for AXP813

As axp20x-battery-power-supply now supports AXP813, add a cell for it.

Signed-off-by: Quentin Schulz <quentin.schulz@bootlin.com>
Reviewed-by: Chen-Yu Tsai <wens@csie.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>