ATTR_CEN_VPD_DRAM_ADDRESS_MIRRORING: Remove writable tag

VPD attributes are read from VPD, but it doesn't make sense
for them to be writable. This fixes compiler issues on hostboot
side as well.

Change-Id: Ic707ea7e61b5d76fc85ff875d93b502adc645726
CQ: SW415651
Reviewed-on: http://ralgit01.raleigh.ibm.com/gerrit1/56128
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: Hostboot CI <hostboot-ci+hostboot@us.ibm.com>
Reviewed-by: LUCAS W. MULKEY <lwmulkey@us.ibm.com>
Reviewed-by: Jennifer A. Stofer <stofer@us.ibm.com>
Reviewed-on: http://ralgit01.raleigh.ibm.com/gerrit1/56135
Reviewed-by: William G. Hoffa <wghoffa@us.ibm.com>

src/import/chips/centaur/procedures/xml/attribute_info/dimm_spd_vpd_attributes.xml

@@ -1821,7 +1821,6 @@ The following attributes are from Centaur VPD.
     </enum>
     <platInit/>
     <array> 2 2</array>
-    <writeable/>
 </attribute>
 
 <!-- Attributes added to support the VPD which was formally using the EFF settings -->