enb_amarisoft: add rf_dev_sync option to specify the sync type

this allows to specify "external_clock" for example to use
the external REF input of an USRP

Change-Id: Ie066993440840cbd2c18e8f0c0c577b92ac7092d
diff --git a/src/osmo_gsm_tester/obj/enb.py b/src/osmo_gsm_tester/obj/enb.py
index 6384f15..fea78de 100644
--- a/src/osmo_gsm_tester/obj/enb.py
+++ b/src/osmo_gsm_tester/obj/enb.py
@@ -34,6 +34,7 @@
         'rx_gain': schema.UINT,
         'rf_dev_type': schema.STR,
         'rf_dev_args': schema.STR,
+        'rf_dev_sync': schema.STR,
         'additional_args[]': schema.STR,
         'inactivity_timer': schema.INT,
         'enable_measurements': schema.BOOL_STR,