ACPI, APEI, Rename CPER and GHES severity constants

The abbreviation of severity should be SEV instead of SER, so the CPER
severity constants are renamed accordingly. GHES severity constants
are renamed in the same way too.

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>