cxl/decoder: Hide physical address information from non-root

Just like /proc/iomem, CXL physical address information is reserved for
root only.

Reviewed-by: Ben Widawsky <ben.widawsky@intel.com>
Reviewed-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Link: https://lore.kernel.org/r/164298416650.3018233.450720006145238709.stgit@dwillia2-desk3.amr.corp.intel.com


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