docs: counter: Consolidate Counter sysfs attributes documentation

Duplicate ABIs are not valid, so let's consolidate these sysfs
attributes into the main sysfs-bus-counter documentation file.

Cc: Patrick Havelange <patrick.havelange@essensium.com>
Reviewed-by: David Lechner <david@lechnology.com>
Signed-off-by: William Breathitt Gray <vilhelm.gray@gmail.com>
Link: https://lore.kernel.org/r/c651ec1c541754ad108160839e2b8425ad089819.1623201081.git.vilhelm.gray@gmail.com


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