ARM: dts: axp209: Add usb_power_supply child node to the ax209 node

Add a node representing the usb power supply part of the axp209 pmic, note
that the usb power supply and the (to be added later) ac power supply will
each have their own child-node, so that they can be separately specified
as power-supply for other nodes using a power-supply property with a
phandle pointing to the right axp209 child-node.

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