drm/panel: simple: Add the 7" DPI panel from Adafruit

This is a basic TFT panel with a 40-pin FPC connector on it.  The
specification doesn't define timings, but the Adafruit instructions
were setting up 800x480 CVT.

v2: Add .bus_format and vsync/hsync flags.

Signed-off-by: Eric Anholt <eric@anholt.net>
Acked-by: Rob Herring <robh@kernel.org>
[treding@nvidia.com: keep entries properly sorted]
Signed-off-by: Thierry Reding <treding@nvidia.com>