us: add "pta" APN for AT&T (bgo #709874)

https://bugzilla.gnome.org/show_bug.cgi?id=709874

Signed-off-by: Thomas Haller <thaller@redhat.com>
diff --git a/serviceproviders.xml b/serviceproviders.xml
index a8397d4..b74aa61 100644
--- a/serviceproviders.xml
+++ b/serviceproviders.xml
@@ -10423,6 +10423,11 @@
 				<usage type="internet"/>
 				<name>Data Connect (old)</name>
 			</apn>
+			<apn value="pta">
+				<plan type="postpaid"/>
+				<usage type="internet"/>
+				<name>LTE</name>
+			</apn>
 		</gsm>
 	</provider>
 	<provider>