ARM: dtsi: axp81x: add USB power supply node

The AXP813/818 has a VBUS power input. Add a device node for it, now
that we support 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: Maxime Ripard <maxime.ripard@bootlin.com>