Input: add D-Link DIR-685 touchkeys driver

This adds support for the D-Link DIR-685 touchkeys found in the
router with this name.

The vendor code calles this a "touchpad" but we are registering
it here under its real name.

Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>