Improve detection and description for MEMVPD_POS issues

Updated attribute description based on changes to requirements
Added explicit check for invalid data to improve debug

Change-Id: I06518a3c7311c1a3ccb4241c9c155e78a067b625
Original-Change-Id: Ie22287166855be1e0d99a52cc4a3ad89d354acfa
Reviewed-on: http://ralgit01.raleigh.ibm.com/gerrit1/32793
Tested-by: Jenkins Server <pfd-jenkins+hostboot@us.ibm.com>
Tested-by: Hostboot CI <hostboot-ci+hostboot@us.ibm.com>
Reviewed-by: Christian R. Geddes <crgeddes@us.ibm.com>
Reviewed-by: Brian R. Silver <bsilver@us.ibm.com>
Reviewed-on: http://ralgit01.raleigh.ibm.com/gerrit1/42150
Tested-by: Jenkins OP Build CI <op-jenkins+hostboot@us.ibm.com>
Tested-by: FSP CI Jenkins <fsp-CI-jenkins+hostboot@us.ibm.com>

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

@@ -79,10 +79,12 @@
     <targetType>TARGET_TYPE_MCS</targetType>
     <description>
       The position of the MCS target's VPD selector data, relative to the EEPROM
-      that contains its data. For systems with a single EEPROM for all chips,
-      this should be equivalent to ATTR_FAPI_POS (the logical MCA position).
+      that contains its data.  The VPD defition supports up to 16 values per
+      EEPROM.
       For systems with an EEPROM per chip, this value should be equivalent to
       ATTR_CHIP_UNIT_POS.
+      For systems with a single EEPROM for all chips, the value should follow
+      the physical position in such a way to fit within the 16 available slots.
     </description>
     <valueType>uint8</valueType>
     <platInit/>