drm/panel: simple: Add support for Rocktech RK070ER9427 LCD panel

This adds support for the Rocktech Display Ltd. RK070ER9427
800(RGB)x480 TFT LCD panel, which can be supported by the
simple panel driver.

Signed-off-by: Jagan Teki <jagan@amarulasolutions.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Thierry Reding <treding@nvidia.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180607134648.2902-1-jagan@amarulasolutions.com