northbridge/amd/agesa/family12/dimmSpd.c: Use ARRAY_SIZE macro

Change-Id: Icf980088c196b152cc4e5e179f7b7e334b695ccc
Signed-off-by: Edward O'Callaghan <eocallaghan@alterapraxis.com>
Reviewed-on: http://review.coreboot.org/7574
Tested-by: build bot (Jenkins)
Reviewed-by: Nicolas Reinecke <nr@das-labor.org>
Reviewed-by: Patrick Georgi <pgeorgi@google.com>
Reviewed-by: Paul Menzel <paulepanter@users.sourceforge.net>