apalis-imx8: turn off lcd backlight before os handover

apalis-imx8: 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 GPIO Apalis BKL1 before booting Linux.

Signed-off-by: Marcel Ziswiler <marcel.ziswiler@toradex.com>
Signed-off-by: Francesco Dolcini <francesco.dolcini@toradex.com>
Reviewed-by: Fabio Estevam <festevam@denx.de>