drm/panel: Add panel driver for Ilitek ILI9806E panel

The Ilitek ILI9806E driver is used in the Pimoroni HyperPixel4
and potentially other displays. Whilst it can support multiple
interfaces, this driver only accounts for SPI configuration and
DPI video data.

Signed-off-by: Dave Stevenson <dave.stevenson@raspberrypi.com>