drm/panel: Add driver for Novatek NT35510-based panels

This adds a driver for panels based on the Novatek NT35510
display driver IC, such as the Hydis HVA40WV1 panel found
in the Samsung GT-S7710.

The NT35510 can be used with both internal and external
backlight (such as GPIO backlight) so we support both:
if no external backlight is found, we register a subdriver
for the internal backlight.

Cc: Stephan Gerhold <stephan@gerhold.net>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20200223121841.26836-3-linus.walleij@linaro.org