drm/panel: Add DT bindings for Novatek NT35510-based panels

This adds device tree bindings for the Novatek NT35510-based
family of panels. Since several such panels are in existence
we define bindings common for all, and define the compatible
string for one certain panel (Hydis HVA40WV1).

As other panels are discovered and investigated, we can add
more compatibles to the binding using oneOf constructions.

Cc: Stephan Gerhold <stephan@gerhold.net>
Cc: devicetree@vger.kernel.org
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20200223121841.26836-2-linus.walleij@linaro.org