ARM: dts: imx51-zii-rdu1: fix touchscreen pinctrl

The pinctrl settings were incorrect for the touchscreen interrupt line, causing
an interrupt storm. This change has been tested with both the atmel_mxt_ts and
RMI4 drivers on the RDU1 units.

The value 0x4 comes from the value of register IOMUXC_SW_PAD_CTL_PAD_CSI1_D8
from the old vendor kernel.

Signed-off-by: Nick Dyer <nick@shmanahar.org>
Fixes: ceef0396

 ("ARM: dts: imx: add ZII RDU1 board")
Cc: <stable@vger.kernel.org> # 4.15+
Reviewed-by: Fabio Estevam <fabio.estevam@nxp.com>
Tested-by: Chris Healy <cphealy@gmail.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>