docs: counter: Update to reflect sysfs internalization

The Counter subsystem architecture and driver implementations have
changed in order to handle Counter sysfs interactions in a more
consistent way. This patch updates the Generic Counter interface
documentation to reflect the changes.

Reviewed-by: David Lechner <david@lechnology.com>
Signed-off-by: William Breathitt Gray <vilhelm.gray@gmail.com>
Link: https://lore.kernel.org/r/91cded13c0145c0d3d0acfe765a2ccb6c9af7c3b.1630031207.git.vilhelm.gray@gmail.com


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