counter: stm32-timer-cnt: remove iio headers

The stm32-timer-cnt driver doesn't use the iio interface. The iio headers
aren't relevant and can be removed as reported by William in [1].
With this change, mod_devicetable.h needs to be added to define the
'of_device_id' struct.

[1] https://lkml.org/lkml/2020/2/10/1516

Reported-by: William Breathitt Gray <[e-mail]>
Signed-off-by: Fabrice Gasnier <[e-mail]>
Signed-off-by: Jonathan Cameron <[e-mail]>

drivers/counter/stm32-timer-cnt.c

···
  *
  */
 #include <linux/counter.h>
-#include <linux/iio/iio.h>
-#include <linux/iio/types.h>
 #include <linux/mfd/stm32-timers.h>
+#include <linux/mod_devicetable.h>
 #include <linux/module.h>
 #include <linux/pinctrl/consumer.h>
 #include <linux/platform_device.h>