Input: add Atmel Captouch Button driver

Add I2C driver for Atmel Capacitive Touch Button device.

Signed-off-by: Hung-yu Wu <hywu@google.com>
Signed-off-by: Grant Grundler <grundler@chromium.org>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>