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 <vilhelm.gray@gmail.com>
Signed-off-by: Fabrice Gasnier <fabrice.gasnier@st.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>