cxl/mem: Add a debugfs version of 'iomem' for DPA, 'dpamem'

Dump the device-physical-address map for a CXL expander in /proc/iomem
style format. E.g.:

  cat /sys/kernel/debug/cxl/mem1/dpamem
  00000000-0fffffff : ram
  10000000-1fffffff : pmem

Reviewed-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Link: https://lore.kernel.org/r/165603885318.551046.8308248564880066726.stgit@dwillia2-xfh
Signed-off-by: Dan Williams <dan.j.williams@intel.com>