mfd: axp20x: Add axp20x-regulator cell for AXP813

Now that axp20x-regulator supports AXP813, we can add a cell for it
to enable it.

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