For example if we need to do 4T4R but have SDR boards with only 2T2R ports, we
need to combine two such boards into single RF port. Allow to do that via
changing SDR schema from taking sdr_dev into accepting sdr_dev_list and
adjusting the code everywhere correspondingly.

For ORS full backward compatibility is preserved via adjusting "ORS mode" proxy
correspondingly.

One example in slapos-render-config.py is changed from 2T2R to be 4T2R on such
RF port combined from two SDR boards. For that example the diff for rendered
enb.cfg is as follows:

    --- a/config/old/enb/iRU1_SDR_tLTE2_tNR/enb.cfg
    +++ b/config/out/enb/iRU1_SDR_tLTE2_tNR/enb.cfg
    @@ -8,7 +8,7 @@
       rf_driver: {
           // __CELL__a__ru 4T2R  (sdr)
           name: "sdr",
    -      args: "dev0=/dev/sdr0",
    +      args: "dev0=/dev/sdr0,dev1=/dev/sdr1",
       },
       tx_gain: [51, 51, 51, 51],
       rx_gain: [52, 52],

All other cases, including ORS, remain without any change.