mx51evk: Fix build error when CONFIG_VIDEO is disabled

The inclusion of LCD patch into mx51evk breaks the build when
CONFIG_VIDEO is disabled. Fix this by splitting the video related
stuff to a new file.

Also rename the function lcd_iomux to setup_iomux_lcd to make the
namings aligned with the other iomux functions.

Signed-off-by: Vikram Narayanan <vikram186@gmail.com>
Signed-off-by: Anatolij Gustschin <agust@denx.de>
Cc: Fabio Estevam <fabio.estevam@freescale.com>
Cc: Stefano Babic <sbabic@denx.de>