ARM: dts: Add binding documentation for AXP20x pmic usb power supply

Add binding documentation for the usb power supply part of the AXP20x pmic.

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Sebastian Reichel <sre@kernel.org>