iio: counter: Add deprecation markings for IIO Counter attributes

The IIO counter subdirectory is now superceded by the Counter subsystem.
This patch adds deprecation warnings to the documentation of the
relevant IIO Counter sysfs attributes.

Acked-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Signed-off-by: William Breathitt Gray <vilhelm.gray@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>