Input: add driver FSL MPR121 capacitive touch sensor

This patch adds basic support for Freescale MPR121 capacitive touch
sensor.  It's an i2c controller with up to 12 capacitance sensing inputs.

Product information (data sheet, application notes) can be found here:
http://www.freescale.com/webapp/sps/site/prod_summary.jsp?code=MPR121



Signed-off-by: Zhang Jiejing <jiejing.zhang@freescale.com>
Signed-off-by: Dmitry Torokhov <dtor@mail.ru>