gpio: Support no-irq stmpe mode

Adds support for boards which have an STMPE GPIO device without the
interrupt pin connected. This means that no interrupt can be received
but the GPIO pins can still be driven and read.

Cc: Grant Likely <grant.likely@secretlab.ca>
Acked-by: Viresh Kumar <viresh.kumar@st.com>
Tested-by: Michel Jaouen <michel.jaouen@stericsson.com>
Signed-off-by: Chris Blair <chris.blair@stericsson.com>
Acked-by: Viresh Kumar <viresh.kumar@st.com>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>