mfd: Fix missing stmpe kerneldoc

Generating kerneldoc for STMPE result in warnings, so fix this by
adding missing documentation.

Signed-off-by: Om Prakash <omprakash.pal@stericsson.com>
Reviewed-by: Rabin Vincent <rabin.vincent@stericsson.com>
Reviewed-by: Jonas Aberg <jonas.aberg@stericsson.com>
Reviewed-by: Srinidhi Kasagar <srinidhi.kasagar@stericsson.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>