vty: add expert commands for MS/BS power control
vty: add expert commands for MS/BS power control

These new commands are useful for debugging MS/BS power control
loops, e.g. one can change power control mode, overwrite the
current BS power reduction or MS power level at run-time.

Change-Id: I1ebb033b02c2bc3b1fa7de874e0035a07297f266
Related: SYS#4918

osmocom/osmo-bts,osmocom/osmo-bts,osmocom/osmo-bts