drm/panel: Add DT bindings for Samsung S6D27A1 display panel

This adds device-tree bindings for the Samsung S6D27A1 RGB
DPI display panel.

Signed-off-by: Markuss Broks <markuss.broks@gmail.com>

v1 -> v2:
changed additionalProperties to unevaluatedProperties;
added vci-supply and vccio-supply as required;

Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20210807133111.5935-2-markuss.broks@gmail.com