Implement a state machine clock that acutally limits how fast

Implement a state machine clock that acutally limits how fast
the nsgige state machine can run. The frequency is of the actual
state transitions, and not the rate of what underlying
instructions might run at.

dev/ns_gige.cc:
    Implement a state machine clock that acutally limits how fast
    the state machine can run.  After each state transition, a
    variable is kept to hold the next state transition until the
    next clock.  The frequency is of the actual state transitions,
    and not the rate of what underlying instructions might run at.
dev/ns_gige.hh:
    Add back the rxKickEvent and txKickEvent events.
python/m5/objects/Ethernet.py:
    Default the state machine clock to '0ns' so the default
    behaviour doesn't change when we actually implement the
    state machine clock.

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extra : convert_revision : 2db1943dee4e91ea75aaee6a91e88f27f01a09dd