dt-bindings: Add vendor prefix for ROCKTECH DISPLAYS LIMITED

Add ROCKTECH DISPLAYS LIMITED (https://rocktech.com.hk

) LCD panel
supplier.

Signed-off-by: Guido Günther <agx@sigxcpu.org>
Signed-off-by: Thierry Reding <treding@nvidia.com>
Link: https://patchwork.freedesktop.org/patch/msgid/b8db03faa344c834e6cc81eef1eca154e1a1bd90.1554114302.git.agx@sigxcpu.org