staging/iio/adc: MXS_LRADC depends on INPUT

mxs-lradc.c uses many input_() functions so it should
depend on INPUT to fix build errors.

drivers/built-in.o: In function `mxs_lradc_ts_unregister':
drivers/staging/iio/adc/mxs-lradc.c:905: undefined reference to `input_unregister_device'
drivers/staging/iio/adc/mxs-lradc.c:905: undefined reference to `input_unregister_device'
drivers/built-in.o: In function `input_report_abs':
include/linux/input.h:399: undefined reference to `input_event'
include/linux/input.h:399: undefined reference to `input_event'
include/linux/input.h:399: undefined reference to `input_event'
drivers/built-in.o: In function `input_report_key':
include/linux/input.h:389: undefined reference to `input_event'
drivers/built-in.o: In function `input_sync':
include/linux/input.h:414: undefined reference to `input_event'
drivers/built-in.o:include/linux/input.h:389: more undefined references to `input_event' follow

Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Acked-by: Marek Vasut <marex@denx.de>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>