dt-bindings: Add Rocktech jh057n00900 panel bindings

The Rocktec jh057n00900 is a 5.5" MIPI DSI video mode panel with a
720x1440 resolution and a built in backlight.

Signed-off-by: Guido Günther <agx@sigxcpu.org>
Signed-off-by: Thierry Reding <treding@nvidia.com>
Link: https://patchwork.freedesktop.org/patch/msgid/ec59b22907ac28764ffb3bec33445b6e019945a4.1554114302.git.agx@sigxcpu.org