dt-bindings: display: sitronix,st7735r: Add Okaya RH128128T

Document support for the Okaya RH128128T display, which is a 128x128
1.44" TFT display driven by a Sitronix ST7715R TFT Controller/Driver.
It can be found on e.g. the Renesas YRSK-LCD-PMOD extension board, which
comes with various Renesas development kits (e.g. Renesas Starter Kit+
for RZ/A1H[1]).

ST7715R and ST7735R are very similar.  Their major difference is that
the former is restricted to displays of up to 132x132 pixels, while the
latter supports displays up to 132x162 pixels.

[1] https://renesasrulz.com/the_vault/f/archive-forum/4981/upgrading-to-the-renesas-rz-a1h

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/20200115124548.3951-3-geert+renesas@glider.be