Add update timestamp property on Logging.Entry

The new UpdateTimestamp property will hold the timestamp of the
most recent change to the log.  When an event log is first created,
this will match the value of the Timestamp property.

As only the Resolved property can change after log creation, this will
in practice indicate when that field was changed.

This is being put in place to match up with an upcoming Redfish event
log field that indicates when that event log was last updated.

Change-Id: Icb1daf2bc35a2362f94d99fbcdd31b9ce93ced2d
Signed-off-by: Matt Spinler <spinler@us.ibm.com>
diff --git a/xyz/openbmc_project/Logging/Entry.interface.yaml b/xyz/openbmc_project/Logging/Entry.interface.yaml
index 5e6cf2e..0f1953e 100755
--- a/xyz/openbmc_project/Logging/Entry.interface.yaml
+++ b/xyz/openbmc_project/Logging/Entry.interface.yaml
@@ -33,6 +33,14 @@
           this error log has been resolved. This will start out 'false' by
           default. Setting this to 'true' will NOT result in the error log being
           deleted.
+    - name: UpdateTimestamp
+      type: uint64
+      description: >
+          The timestamp when any property was last modified, in milliseconds
+          since the epoch (1 Jan 1970 00:00:00 UTC).  As only the
+          Resolved property has a reason to change, a value that doesn't
+          match the Timestamp property's value (which indicates when the
+          log was created) would indicate that Resolved was changed.
 
 enumerations:
     - name: Level