ARM: at91/dt: split sama5d3 lcd pin definitions to match RGB mode configs

The HLCDC (HLCD Controller) IP supports 4 different output mode (RGB444,
RGB565, RGB666 and RGB888) and the pin muxing will depend on the chosen
RGB mode.

Split pin definitions to be able to set pin config according to the
selected mode.

Signed-off-by: Boris Brezillon <boris.brezillon@free-electrons.com>
Tested-by: Anthony Harivel <anthony.harivel@emtrion.de>
Signed-off-by: Nicolas Ferre <nicolas.ferre@atmel.com>