arm64: dts: imx8mq-librem5: add pinctrl for the touchscreen description

In order for the touchscreen interrupt line to work, describe it properly.
Otherwise it can work if defaults are ok, but we cannot be sure.

Fixes: 8f0216b0

 ("arm64: dts: Add a device tree for the Librem 5 phone")
Signed-off-by: Martin Kepplinger <martin.kepplinger@puri.sm>
Reviewed-by: Krzysztof Kozlowski <krzk@kernel.org>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>