Introduced configuration of RU IF offset from configuration file

- the RU now supports IF equipment with custom FR1 TX/RX frequencies
- this configuration parameter together with the if_freq parameter allows
  to set a custom IF RX frequency for the gNB
- if the IF offset is not specified, legacy code applies

openair1/PHY/defs_RU.h

@@ -624,6 +624,8 @@ typedef struct RU_t_s {
   int wakeup_L1_sleep_cnt_max;
   /// DL IF frequency in Hz
   uint64_t if_frequency;
+  /// UL IF frequency offset to DL IF frequency in Hz
+  int if_freq_offset;
 } RU_t;