ACPI, APEI, Document for APEI

Add document for APEI, including kernel parameters and EINJ debug file
sytem interface.

Signed-off-by: Huang Ying <ying.huang@intel.com>
Signed-off-by: Andi Kleen <ak@linux.intel.com>
Signed-off-by: Len Brown <len.brown@intel.com>