modem-3gpp: allow loading and changing EPS UE mode of operation

The UE modes of operation for LTE are defined in 3GPP TS 24.301 (e.g.
section 4.3 in v10.3.0):
  * PS mode 1: EPS only, 'voice centric'
  * PS mode 2: EPS only, 'data centric'
  * CS/PS mode 1: EPS and non-EPS, 'voice centric'
  * CS/PS mode 2: EPS and non-EPS, 'data centric'

The mode specifies, among other things, how the UE should behave w.r.t
CS fallback depending on the capabilities reported by the network.

introspection/org.freedesktop.ModemManager1.Modem.Modem3gpp.xml

@@ -85,6 +85,16 @@
       <arg name="results" type="aa{sv}" direction="out" />
     </method>
 
+    <!--
+        SetEpsUeModeOperation:
+        @mode: a <link linkend="MMModem3gppEpsUeModeOperation">MMModem3gppEpsUeModeOperation</link>.
+
+        Sets the UE mode of operation for EPS.
+    -->
+    <method name="SetEpsUeModeOperation">
+      <arg name="mode" type="u" direction="in" />
+    </method>
+
     <!--
         Imei:
 
@@ -145,5 +155,14 @@
     -->
     <property name="SubscriptionState" type="u" access="read" />
 
+    <!--
+        EpsUeModeOperation:
+
+        A <link linkend="MMModem3gppEpsUeModeOperation">MMModem3gppEpsUeModeOperation</link>
+        value representing the UE mode of operation for EPS, given as an unsigned integer
+        (signature <literal>"u"</literal>).
+    -->
+    <property name="EpsUeModeOperation" type="u" access="read" />
+
   </interface>
 </node>