mx6sabresd: Allow LVDS backlight to be functional after a kernel reboot

mx6sabresd: Allow LVDS backlight to be functional after a kernel reboot

Currently the LVDS backlight does not work in U-Boot after a “reboot” comand
in the kernel.

This problem occurs because the kernel uses this pin in PWM mode and U-Boot
does not configure the backlight pin as GPIO functionality.

So fix the problem by explicitly configuring the backlight pin as GPIO in
U-Boot.

Signed-off-by: Marco Franchi <marco.franchi@nxp.com>
Acked-by: Stefano Babic <sbabic@denx.de>
Tested-by: Fabio Estevam <fabio.estevam@nxp.com>