counter: 104-quad-8: Documentation: Add Generic Counter sysfs documentation

This patch adds standard documentation for the Generic Counter interface
userspace sysfs attributes of the 104-QUAD-8 driver.

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>