mfd: axp20x: Add USB power supply mfd cell to AXP813

The AXP813 has a VBUS power input. Now that the axp20x_usb_power driver
supports this variant, we can add an mfd cell for it to use it.

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