staging/fbtft: Add support for display variants

Display variants are intended as a replacement for the now-deleted
fbtft_device drivers. Drivers can register additional compatible
strings with a custom callback that can make the required changes
to the fbtft_display structure.

Start the ball rolling by adding adafruit18, adafruit18_green and
sainsmart18 displays.

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