Input: mpr121 - switch to device tree probe

This driver currently only supports legacy platform data probe.  This
change adds device tree support and gets rid of platform data probe code
since no one is actually using mpr121 platform data in the mainline.

The device tree property parsing code is based on the work of
atmel_captouch driver.

Signed-off-by: Akinobu Mita <akinobu.mita@gmail.com>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>