enb_amarisoft: add option to enable and configure DL AWGN channel

expose option to enable DL AWGN noise and make SNR configurable

Change-Id: Ib248e87e5abd855d4c974eab42ec40828c6e1d40
diff --git a/sysmocom/defaults.conf b/sysmocom/defaults.conf
index 2c99e04..dd97456 100644
--- a/sysmocom/defaults.conf
+++ b/sysmocom/defaults.conf
@@ -113,6 +113,8 @@
   transmission_mode: 1
   num_cells: 1
   enable_measurements: false
+  enable_dl_awgn: false
+  dl_awgn_snr: 30
   a1_report_type: rsrp
   a1_report_value: -105
   a1_hysteresis: 0