MFC: r201395

MFC: r201395

- Preserve the PROM IOMMU in order to allow OFW drivers to continue to
  work.
- Sanity check the parameters passed to the implementations of the
  pcib_{read,write}_config() methods. Using illegal values can cause
  no real harm but it doesn't hurt to avoid unnecessary data error
  traps requiring to flush and re-enable the level 1 caches.

git-svn-id: svn://svn.freebsd.org/base/stable/8@202400 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f