ARM: dts: socfpga: Remove unneeded reg from stmpe_touchscreen

The stmpe_touchscreen node in Cyclone5 MCV EVK has a reg property, but
this is not used by the driver. Moreover the binding documentation do
not define this property. Having a reg property without a unit name will
trigger the following warning when compiled with W=1:

Node /soc/i2c@ffc04000/stmpe811@41/stmpe_touchscreen has a reg or ranges
property, but no unit name

Remove the superfluous reg property.

Signed-off-by: Florian Vaussard <florian.vaussard@heig-vd.ch>
Signed-off-by: Dinh Nguyen <dinguyen@kernel.org>