counter: Documentation: Add Generic Counter sysfs documentation

This patch adds standard documentation for the userspace sysfs
attributes of the Generic Counter interface.

Reviewed-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>