board: toradex: turn off lcd backlight before OS handover

U-Boot typically tears down the display controller before handing
control over to Linux. On LCD displays disabling pixel clock leads to a
fading out effect with vertical/horizontal lines. Make sure to disable
back light before booting Linux.

Signed-off-by: Gerard Salvatella <gerard.salvatella@toradex.com>
Signed-off-by: Philippe Schenker <philippe.schenker@toradex.com>
Signed-off-by: Tom Warren <twarren@nvidia.com>