staging: fbtft: add support for C-Berry28 display

This patch adds support for the C-Berry28, a small 2.8" color display
with a resolution of 320x240 pixels from admatec for the Raspberry Pi.
It uses the Sitronix ST7789V display controller along with a custom
init sequence and custom gamma curves.

Signed-off-by: Dennis Menschel <menschel-d@posteo.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>