drm/panel: Add support for ARM Versatile panels

The ARM reference designs "Versatile AB" and "Versatile PB"
contain panel connectors with autodetection of the connected
panel type. This adds a small driver utilizing the MFD syscon
look-up to read the autodetection register and set up the
corresponding panel appropriately.

In the source file there is a bit of elaboration of the
panel types and interfaces on these boards.

This was tested with the PL111 DRM driver on the ARM Versatile
AB with the IB2 daughterboard.

Reviewed-by: Eric Anholt <eric@anholt.net>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20180205192013.5349-2-linus.walleij@linaro.org