dt-bindings: Add Logic PD Type 28 display panel

This patch adds documentation of device tree bindings for the WVGA panel
Logic PD Type 28 display.

Signed-off-by: Adam Ford <aford173@gmail.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20191016135147.7743-2-aford173@gmail.com