ARM/fb: ep93xx: switch framebuffer to use modedb only

All the EP93xx boards exclusively use modedb to look up video
modes from the command line. Root out the parametrization of
custom video modes from the platform data and board files
and simplify the driver.

Cc: Jean-Christophe Plagniol-Villard <plagnioj@jcrosoft.com>
Reviewed-by: H Hartley Sweeten <hsweeten@visionengravers.com>
Acked-by: Tomi Valkeinen <tomi.valkeinen@ti.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Olof Johansson <olof@lixom.net>