ARM: dts: axp22x.dtsi: Add usb_power_supply node

Add a node describing the (optional) usbpower-supply of the
axp221 / axp223 pmic.

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>