cxl/region: Delete 'region' attribute from root decoders

For switch and endpoint decoders the relationship of decoders to regions
is 1:1. However, for root decoders the relationship is 1:N. Also,
regions are already children of root decoders, so the 1:N relationship
is observed by walking the following glob:

    /sys/bus/cxl/devices/$decoder/region*

Hide the vestigial 'region' attribute for root decoders.

Reviewed-by: Vishal Verma <vishal.l.verma@intel.com>
Reviewed-by: Alison Schofield <alison.schofield@intel.com>
Link: https://lore.kernel.org/r/165853776328.2430596.4647259305040072751.stgit@dwillia2-xfh.jf.intel.com
Signed-off-by: Dan Williams <dan.j.williams@intel.com>