Input: ad7879 - drop platform data support

This driver supports configuration via platform data but
absolutely nothing in the upstream kernel uses it. Since this
configuration allows harmful practices such as encoding the
GPIO base for the chip, delete platform data support so that
no new platform using it gets introduced.

Also: include the right driver header, not <linux/gpio.h>.

Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Acked-by: Michael Hennerich <michael.hennerich@analog.com>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>