dt-bindings: input: touchscreen: add minimum pressure touchscreen property

Add a common touchscreen optional property that will specify
the minimum pressure applied to the screen that is needed
such that the driver will report the touch event.

Signed-off-by: Eugen Hristev <eugen.hristev@microchip.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>