fix xp=1 option and added RUNMODEM description for MIMO configuration

ci-scripts/conf_files/gNB_SA_n78_106PRB.2x2_usrpn310.conf

ci-scripts/conf_files/gNB_SA_n78_133PRB.2x2_usrpn310.conf

...cripts/conf_files/gnb.band78.nsa_2x2.106PRB.usrpn310.conf

...pts/conf_files/gnb.band78.sa.fr1.106PRB.2x2.usrpn310.conf

...pts/conf_files/gnb.band78.sa.fr1.162PRB.2x2.usrpn310.conf

doc/RUNMODEM.md

doc/images/mimo_antenna_ports.png

openair2/LAYER2/NR_MAC_gNB/config.c

openair2/LAYER2/NR_MAC_gNB/gNB_scheduler_dlsch.c

openair2/LAYER2/NR_MAC_gNB/gNB_scheduler_primitives.c

openair2/LAYER2/NR_MAC_gNB/mac_proto.h

openair2/LAYER2/NR_MAC_gNB/nr_mac_gNB.h