counter: stm32-lptimer-cnt: remove iio counter abi

Currently, the STM32 LP Timer counter driver registers into both IIO and
counter subsystems, which is redundant.

Remove the IIO counter ABI and IIO registration from the STM32 LP Timer
counter driver since it's been superseded by the Counter subsystem
as discussed in [1].

Keep only the counter subsystem related part.
Move a part of the ABI documentation into a driver comment.

This also removes a duplicate ABI warning
$ scripts/get_abi.pl validate
...
/sys/bus/iio/devices/iio:deviceX/in_count0_preset is defined 2 times:
  ./Documentation/ABI/testing/sysfs-bus-iio-timer-stm32:100
  ./Documentation/ABI/testing/sysfs-bus-iio-lptimer-stm32:0

[1] https://lkml.org/lkml/2021/1/19/347



Acked-by: William Breathitt Gray <vilhelm.gray@gmail.com>
Signed-off-by: Fabrice Gasnier <fabrice.gasnier@foss.st.com>
Link: https://lore.kernel.org/r/1611926542-2490-1-git-send-email-fabrice.gasnier@foss.st.com


Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>