dmaengine: idxd: Add ABI documentation for shared wq

Add the sysfs attribute bits in ABI/stable for shared wq support.

Signed-off-by: Jing Lin <jing.lin@intel.com>
Signed-off-by: Dave Jiang <dave.jiang@intel.com>
Reviewed-by: Tony Luck <tony.luck@intel.com>
Reviewed-by: Dan Williams <dan.j.williams@intel.com>
Link: https://lore.kernel.org/r/160382008649.3911367.10851752182908509837.stgit@djiang5-desk3.ch.intel.com
Signed-off-by: Vinod Koul <vkoul@kernel.org>