ARM: at91/dt: add alternative pin muxing for sama5d3 lcd pins

Define alternative pin muxing for the LCDC pins.

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>