iio:adc: at91 requires the input subsystem

Building the at91 adc driver with CONFIG_INPUT disabled results in this
build error:

ERROR: "input_event" [drivers/iio/adc/at91_adc.ko] undefined!
ERROR: "input_unregister_device" [drivers/iio/adc/at91_adc.ko] undefined!
ERROR: "input_free_device" [drivers/iio/adc/at91_adc.ko] undefined!
ERROR: "input_register_device" [drivers/iio/adc/at91_adc.ko] undefined!
ERROR: "input_set_abs_params" [drivers/iio/adc/at91_adc.ko] undefined!
ERROR: "input_allocate_device" [drivers/iio/adc/at91_adc.ko] undefined!

To make sure we can build random configurations, this adds a Kconfig
dependency on CONFIG_INPUT, as we do for other similar drivers.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Cc: linux-iio@vger.kernel.org
Cc: Josh Wu <josh.wu@atmel.com>
Cc: Maxime Ripard <maxime.ripard@free-electrons.com>
Acked-by: Nicolas Ferre <nicolas.ferre@atmel.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>