{enb,ms}_srs: Add DL QAM-256 support

Due to the integration of DL-QAM256 another table for DL max rates is needed.
Therefore, I added the parameter 'qam256' to the feature list in the resource.cfg.

The patch also enables the correct UE settings in the config file.

Change-Id: I2d34395449cdcfb31db66ea887d9adbee551e757
diff --git a/src/osmo_gsm_tester/core/schema.py b/src/osmo_gsm_tester/core/schema.py
index 72c2738..816f439 100644
--- a/src/osmo_gsm_tester/core/schema.py
+++ b/src/osmo_gsm_tester/core/schema.py
@@ -111,7 +111,7 @@
     raise ValueError('Unknown Cipher value: %r' % val)
 
 def modem_feature(val):
-    if val in ('sms', 'gprs', 'voice', 'ussd', 'sim', '2g', '3g', '4g'):
+    if val in ('sms', 'gprs', 'voice', 'ussd', 'sim', '2g', '3g', '4g', 'dl_qam256'):
         return True
     raise ValueError('Unknown Modem Feature: %r' % val)