ARM: dts: imx6sl-evk: add missing GPIO iomux setting

On i.MX6SL EVK board, the MX6SL_PAD_KEY_ROW5 pin is
used as lcd 3v3 regulator control pin, need to make
sure MX6SL_PAD_KEY_ROW5 is muxed as GPIO function
for controlling lcd 3v3 regulator.

Signed-off-by: Anson Huang <Anson.Huang@nxp.com>
Reviewed-by; Fabio Estevam <fabio.estevam@nxp.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>