northbridge/amd/agesa/agesawrapper_call.h: Decode status codes

Decode obscure AGESA status codes into their respective string forms.

Change-Id: Iccf175ef62e5005af6ebbfb1bd0acec8aedc2eaa
Signed-off-by: Edward O'Callaghan <eocallaghan@alterapraxis.com>
Reviewed-on: http://review.coreboot.org/6402
Tested-by: build bot (Jenkins)
Reviewed-by: Paul Menzel <paulepanter@users.sourceforge.net>
Reviewed-by: Bruce Griffith <Bruce.Griffith@se-eng.com>