Input: stmpe-ts - report BTN_TOUCH event

stmpe touchscreen supports BTN_TOUCH event but doesn't report it. Add
BTN_TOUCH reporting capability to the stmpe touchscreen driver.

Signed-off-by: Vipul Kumar Samar <vipulkumar.samar@st.com>
Reviewed-by: Viresh Kumar <viresh.kumar@linaro.org>
Acked-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>