video: atmel_lcdfb: add device tree suport

get display timings from device tree
Use videomode helpers to get display timings and configurations from
device tree

Signed-off-by: Jean-Christophe PLAGNIOL-VILLARD <plagnioj@jcrosoft.com>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>