arm64: dts: imx: Fix imx8*-var-som touchscreen property sizes

The common touchscreen properties are all 32-bit, not 16-bit. These
properties must not be too important as they are all ignored in case of an
error reading them.

Signed-off-by: Rob Herring <robh@kernel.org>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>