ARM: dts: imx6qdl-sabresd: Configure the ECSPI1 chip select pin

GPIO4_9 is used as ECSPI1 chip select and it needs to be configured as GPIO.

Configure the pin functionality explicitly in the dts file instead of relying
on the fact that it comes configured as GPIO from POR or from the bootloader.

Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>
Signed-off-by: Shawn Guo <shawn.guo@freescale.com>