Add support for locking radio registers (1/n)

[antoinevg]

This PR adds support for locking radio registers.

Usage:

    # Lock the RADIO_FREQUENCY_RF register
    hackrf_debug --radio --register 1 --lock 1
    
    # Unlock the RADIO_FREQUENCY_RF register
    hackrf_debug --radio --register 1 --lock 0

Implementation decisions:

• We don't have multi-field registers so locks are coarse-grained over the entire register.
• Attempting to write to a locked register returns an error: RADIO_ERR_LOCKED_REGISTER
• A lock prevents changing the register value rather than the application of a register value. This is to avoid a situation where registers are locked, other host software makes unanticipated changes and then there is a potentially unknown value in the register when it is unlocked.
• Locks apply independent of the selected bank rather than per-bank. i.e. You can't have a register which is locked for RX but unlocked for APPLIED, ACTIVE, IDLE, TX and ALL.
• Calling radio_init() clears all register locks.

[martinling]

@miek and I were just discussing this but we don't really understand the use case yet - can you explain a bit?