input: Add STMPE keypad driver

Add an input driver for the keypad on STMPE I/O expanders.  This driver
uses the common support provided by the STMPE MFD driver.

Acked-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>
Acked-by: Linus Walleij <linus.walleij@stericsson.com>
Signed-off-by: Rabin Vincent <rabin.vincent@stericsson.com>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>