drm: add DT bindings documentation for atmel-hlcdc-dc driver

The Atmel HLCDC (HLCD Controller) IP available on some Atmel SoCs (i.e.
at91sam9n12, at91sam9x5 family or sama5d3 family) provides a display
controller device.

The HLCDC block provides a single RGB output port, and only supports LCD
panels connection to LCD panels for now.

The atmel,panel property link the HLCDC RGB output with the LCD panel
connected on this port (note that the HLCDC RGB connector implementation
makes use of the DRM panel framework).

Connection to other external devices (DRM bridges) might be added later by
mean of a new atmel,xxx (atmel,bridge) property.

Signed-off-by: Boris Brezillon <boris.brezillon@free-electrons.com>
Acked-by: Nicolas Ferre <nicolas.ferre@atmel.com>