cxl: Add decoders_committed sysfs attribute to cxl_port

cxl: Add decoders_committed sysfs attribute to cxl_port

This attribute allows cxl-cli to determine whether there are decoders
committed to a memdev.  This is only a snapshot of the state, and
doesn't offer any protection or serialization against a concurrent
disable-region operation.

Reviewed-by: Jim Harris <jim.harris@samsung.com>
Suggested-by: Dan Williams <dan.j.williams@intel.com>
Signed-off-by: Dave Jiang <dave.jiang@intel.com>
Link: https://lore.kernel.org/r/169747907439.272156.10261062080830155662.stgit@djiang5-mobl3
Signed-off-by: Dan Williams <dan.j.williams@intel.com>