staging: fbtft: Add minipitft13 variant

The Adafruit Mini-PiTFT13 display needs offsets applying when rotated,
so use the "variant" mechanism to select a custom set_addr_win method
using a dedicated compatible string of "fbtft,minipitft13".

See: https://github.com/raspberrypi/firmware/issues/1524



Signed-off-by: Phil Elwell <phil@raspberrypi.com>