drm/panel: db7430: Add driver for Samsung DB7430

This adds a new driver for the Samsung DB7430 DPI display
controller as controlled over SPI.

Right now the only panel product we know that is using this
display controller is the LMS397KF04 but there may be more.

This is the first regular panel driver making use of the
MIPI DBI helper library. The DBI "device" portions can not
be used because that code assumes the use of a single
regulator and specific timings around the reset pulse that
do not match the DB7430 datasheet.

Cc: Paul Cercueil <paul@crapouillou.net>
Cc: Doug Anderson <dianders@chromium.org>
Acked-by: Noralf Trønnes <noralf@tronnes.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Reviewed-by: Douglas Anderson <dianders@chromium.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20210610220527.366432-1-linus.walleij@linaro.org