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>