sunxi: gpio: Add temporary implementation of name_to_gpio()

Until sunxi moves to device tree (e.g. for USB) we need to convert named
GPIOs to numbers. Add a function to do this.

This fixes the USB / EHCI support not working on the LinkSprite pcDuino3
(which uses devicemodel).

Signed-off-by: Simon Glass <sjg@chromium.org>
Acked-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>