Support added for 2 RX Antenna case in ulsim and fixed indexing issue in gNB RX chain

Issue seen in nr_ulsim Test case for 2 RX Antenna case. Here under function nr_ulsch_extract_rbs_single() :- Buffer rxF_ext and ul_ch0_ext was getting corrupted due to index rxF_ext_index, ul_ch0_ext_index and ul_ch0_index were not reset for the second RX antenna loop. Also modified the nr_ulsim Test case to support Two RX Antenna and added support via command line arguments.