x86, e820: add support for AddressRangeUnusuable ACPI memory type

Add support for the E820_UNUSABLE memory type, which is defined in
Revision 3.0b (Oct.  10, 2006) of the ACPI Specification on p.  394 Table
14-1:

  AddressRangeUnusuable This range of address contains memory in which
  errors have been detected.  This range must not be used by the OSPM.

Signed-off-by: Joseph Cihula <joseph.cihula@intel.com>
Signed-off-by: Shane Wang <shane.wang@intel.com>
Signed-off-by: Gang Wei <gang.wei@intel.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Ingo Molnar <mingo@elte.hu>