Change ATTR_MSS_WR_VREF_OFFSET to be override-only

Change-Id: I11873b48202c083fadecaae67b6e15ae1d97c783
Reviewed-on: http://ralgit01.raleigh.ibm.com/gerrit1/52027
Reviewed-by: STEPHEN GLANCY <sglancy@us.ibm.com>
Tested-by: FSP CI Jenkins <fsp-CI-jenkins+hostboot@us.ibm.com>
Tested-by: Jenkins Server <pfd-jenkins+hostboot@us.ibm.com>
Tested-by: HWSV CI <hwsv-ci+hostboot@us.ibm.com>
Tested-by: Hostboot CI <hostboot-ci+hostboot@us.ibm.com>
Reviewed-by: ANDRE A. MARIN <aamarin@us.ibm.com>
Reviewed-by: Jennifer A. Stofer <stofer@us.ibm.com>
Reviewed-on: http://rchgit01.rchland.ibm.com/gerrit1/69812
Reviewed-by: Sachin Gupta <sgupta2m@in.ibm.com>
Tested-by: Sachin Gupta <sgupta2m@in.ibm.com>

src/import/chips/p9/procedures/xml/attribute_info/memory_mcs_attributes.xml

@@ -3311,8 +3311,10 @@
       </description>
       <default>0</default>
       <valueType>int8</valueType>
-      <writeable/>
       <platInit/>
+      <overrideOnly/>
+      <initToZero/>
+      <writeable/>
       <mssAccessorName>wr_vref_offset</mssAccessorName>
     </attribute>