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counter driver since it's been superseded by the Counter subsystem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>
diff --git a/drivers/counter/Kconfig b/drivers/counter/Kconfig
index 2de53ab..cbdf842 100644
--- a/drivers/counter/Kconfig
+++ b/drivers/counter/Kconfig
@@ -41,7 +41,7 @@
 
 config STM32_LPTIMER_CNT
 	tristate "STM32 LP Timer encoder counter driver"
-	depends on (MFD_STM32_LPTIMER || COMPILE_TEST) && IIO
+	depends on MFD_STM32_LPTIMER || COMPILE_TEST
 	help
 	  Select this option to enable STM32 Low-Power Timer quadrature encoder
 	  and counter driver.