Add support for the Okaya RH128128T display to the st7735r driver on
DT-enabled systems.

The RH128128T is a 128x128 1.44" TFT display driven by a Sitronix
ST7715R TFT Controller/Driver.  The latter is very similar to the
ST7735R, and can be handled by the existing st7735r driver.
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Sam Ravnborg <sam@ravnborg.org>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20200115124548.3951-6-geert+renesas@glider.be