drm/panel: Add support for the Raspberry Pi 7" Touchscreen.

drm/panel: Add support for the Raspberry Pi 7" Touchscreen.

This driver communicates with the Atmel microcontroller for sequencing
the poweron of the TC358762 DSI-DPI bridge and controlling the
backlight PWM.

v2: Set the same default orientation as the closed source firmware
    used, which is the best for viewing angle.
v3: Rewrite as an i2c client driver after bridge driver rejection.
v4: Finish probe without the DSI host, using the new delayed
    registration, and attach to the host during mipi_dsi_driver probe.
v5: Rework to drop the "probe without DSI host" mode again, now that
    vc4 will create the host early on.
v6: Drop unused brightness #define (noticed by Thierry)

Signed-off-by: Eric Anholt <eric@anholt.net>
Link: https://patchwork.freedesktop.org/patch/msgid/20170927193654.12609-4-eric@anholt.net
Reviewed-by: Archit Taneja <architt@codeaurora.org>
Acked-by: Thierry Reding <treding@nvidia.com>