x86: fix /dev/mem compatibility under PAT

Add ioremap_default(), which gives a sane mapping without worrying about
type conflicts.

Use it in /dev/mem read in place of ioremap(), as with ioremap(),
any mapping of the region (other than UC_MINUS) will cause a conflict
and failure of /dev/mem read.

Should address the vbetest failure reported at:

  http://bugzilla.kernel.org/show_bug.cgi?id=11057

Signed-off-by: Venkatesh Pallipadi <venkatesh.pallipadi@intel.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>