dt-bindings: panel: Add compatible for Xingbangda XBD599 panel

Xingbangda XBD599 is a 5.99" 720x1440 MIPI-DSI LCD panel. It is based on
Sitronix ST7703 LCD controller just like rocktech,jh057n00900. It is
used in PinePhone.

Add a compatible for it.

Signed-off-by: Ondrej Jirman <megous@megous.com>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20200701162928.1638874-4-megous@megous.com