This adds a driver for the usb power_supply bits of the axp20x PMICs.
I initially started writing my own driver, before coming aware of
Bruno Prémont's excellent earlier RFC with a driver for this.
My driver was lacking CURRENT_MAX and VOLTAGE_MIN support Bruno's
drvier has, so I've copied the code for those from his driver.
Note that the AC-power-supply and battery charger bits will need separate
drivers. Each one needs its own devictree child-node so that other
devicetree nodes can reference the right power-supply, and thus each one
will get its own mfd-cell / platform_device and platform-driver.
Cc: Bruno Prémont <bonbons@linux-vserver.org> Acked-by: Lee Jones <lee.jones@linaro.org> Signed-off-by: Bruno Prémont <bonbons@linux-vserver.org> Signed-off-by: Hans de Goede <hdegoede@redhat.com> Signed-off-by: Sebastian Reichel <sre@kernel.org>