mfd: Add support for stmpe variant 610

STMPE610 is very much like STMPE811, except the number of gpio pins, which is 8
in 811 and 6 in 610. This patch adds support for variant 610. STMPE610 will
share most of the code with STMPE811.

Signed-off-by: Viresh Kumar <viresh.kumar@st.com>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>