DT: video: atmel_lcdc: Add example of fixed framebuffer memory

This drivers allows a fixed framebuffer memory to be set by an additional
IORESOURCE_MEM resource. Thus add an example to the DT documentation.

Signed-off-by: Alexander Stein <alexander.stein@systec-electronic.com>
Signed-off-by: Nicolas Ferre <nicolas.ferre@atmel.com>