mfd: Add support for irq over gpio pin to stmpe

On many boards, stmpe is present as an separate device (not as part of SoC).
Here gpio lines are mostly used for getting interrupts. This patch adds in
support to handle irq over gpio pin.

Signed-off-by: Viresh Kumar <viresh.kumar@st.com>
Acked-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>