Tools: add known Calibration tag to apm.pdef.xml

this will allow a GCS to hide anything that a user should never
ordinarily have to see in terms of calibration parameters

Tools/autotest/param_metadata/param.py

@@ -31,6 +31,7 @@ known_param_fields = [
              'Bitmask',
              'Volatile',
              'ReadOnly',
+             'Calibration',
                       ]
 
 # Follow SI units conventions from:

Tools/autotest/param_metadata/xmlemit.py

@@ -46,6 +46,9 @@ class XmlEmit(Emit):
             if hasattr(param, 'User'):
                 t += ' user=%s' % quoteattr(param.User)  # i.e. Standard or Advanced
 
+            if hasattr(param, 'Calibration'):
+                t += ' calibration=%s' % quoteattr(param.Calibration)  # i.e. Standard or Advanced
+
             t += ">\n"
 
             # Add values as chidren of this node