cxl/Documentation: List attribute permissions

Clarify the access permission of CXL sysfs attributes in the
documentation to help development of userspace tooling.

Reported-by: Alison Schofield <alison.schofield@intel.com>
Reviewed-by: Alison Schofield <alison.schofield@intel.com>
Reviewed-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Link: https://lore.kernel.org/r/165603881198.551046.12893348287451903699.stgit@dwillia2-xfh


Signed-off-by: Dan Williams <dan.j.williams@intel.com>