Input: add driver for Microchip's CAP1106

This patch adds a driver for Microchips CAP1106, an I2C driven, 6-channel
capacitive touch sensor.

For now, only the capacitive buttons are supported, and no specific
settings that can be tweaked for individual channels, except for the
device-wide sensitivity gain. The defaults seem to work just fine out of
the box, so I'll leave configurable parameters for someone who's in need
of them and who can actually measure the impact. All registers are
prepared, however. Many of them are just not used for now.

The implementation does not make any attempt to be compatible to platform
data driven boards, but fully depends on CONFIG_OF.

Power management functions are also left for volounteers with the ability
to actually test them.

Signed-off-by: Daniel Mack <zonque@gmail.com>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>