SMF: Make UNSECURE_HOMER_ADDR unwriteable on FAPI

Change-Id: I8c7fa38b2b1bc57470fe0fe0de518efed0d0002b
Reviewed-on: http://rchgit01.rchland.ibm.com/gerrit1/69582
Reviewed-by: Daniel M. Crowell <dcrowell@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: PPE CI <ppe-ci+hostboot@us.ibm.com>
Reviewed-by: Prem Shanker Jha <premjha2@in.ibm.com>
Tested-by: Hostboot CI <hostboot-ci+hostboot@us.ibm.com>
Tested-by: HWSV CI <hwsv-ci+hostboot@us.ibm.com>
Reviewed-by: Gregory S. Still <stillgs@us.ibm.com>
Reviewed-on: http://rchgit01.rchland.ibm.com/gerrit1/69589
Reviewed-by: RAJA DAS <rajadas2@in.ibm.com>

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

@@ -2598,7 +2598,6 @@
     Consumers: Read by p9_hcode_image_build.C
     </description>
     <valueType>uint64</valueType>
-    <writeable/>
     <initToZero/>
   </attribute>
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