Add const to the end of spd decoder methods to denote unchanged mem vars

Change-Id: Ic2934bf3bc7a822216260cb892e0c49c0730536f
Original-Change-Id: I6be84549a46484d9d3d4ad3212e624b4cbbd6e07
Reviewed-on: http://ralgit01.raleigh.ibm.com/gerrit1/38875
Tested-by: Jenkins Server <pfd-jenkins+hostboot@us.ibm.com>
Tested-by: Hostboot CI <hostboot-ci+hostboot@us.ibm.com>
Reviewed-by: STEPHEN GLANCY <sglancy@us.ibm.com>
Reviewed-by: JACOB L. HARVEY <jlharvey@us.ibm.com>
Reviewed-by: Louis Stermole <stermole@us.ibm.com>
Reviewed-by: Thi N. Tran <thi@us.ibm.com>
Reviewed-by: Jennifer A. Stofer <stofer@us.ibm.com>
Reviewed-on: http://ralgit01.raleigh.ibm.com/gerrit1/54757
CI-Ready: Daniel M. Crowell <dcrowell@us.ibm.com>
Reviewed-by: Daniel M. Crowell <dcrowell@us.ibm.com>
Tested-by: Daniel M. Crowell <dcrowell@us.ibm.com>