mfd: stmpe: Use devm_*() routines

This patch frees stmpe driver from tension of freeing resources.
devm_* derivatives of multiple routines are used while allocating resources,
which would be freed automatically by kernel.

Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>
Acked-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>