parisc: sba_iommu: Fix build warning if procfs if disabled

Clean up the code, e.g. make proc_mckinley_root static, drop the now
empty mckinley header file and remove some unneeded ifdefs around procfs
functions.

Signed-off-by: Helge Deller <deller@gmx.de>
Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202308300800.Jod4sHzM-lkp@intel.com/
Fixes: 77e0ddf0 ("parisc: ccio-dma: Create private runway procfs root entry")