Add option to use global noise power for all RX channels in RFSimulator

Added new command line option to rfsimulator: noise_power_dBFS. This options allows the user to configure noise power in dBFS added instead of the per-channel value.

This makes it so noise is not accumulated per channel reaching higher than expected values.

The dBFS unit allows easy configuration. The gNB by default uses -36 dBFS setting. e.g. to reach 3dB SNR one should set the noise_power_dBFS value to -39.