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.

The following lines are required in config.txt, to keep the firmware
from trying to bash our I2C lines and steal the DSI interrupts:

    disable_touchscreen=1
    ignore_lcd=2
    mask_gpu_interrupt1=0x1000

This means that the firmware won't power on the panel at boot time (no
rainbow) and the touchscreen input won't work.  The native input
driver for the touchscreen still needs to be written.

v2: Set the same default orientation as the closed source firmware
    used, which is the best for viewing angle.

Signed-off-by: Eric Anholt <eric@anholt.net>