Better handling of latency/frequency parameter types

Better handling of latency/frequency parameter types

python/m5/config.py:
    Addr is slightly different from memory size in that Addr
    will take non strings.
    Deal with the fact that the convert.toFoo functions only accept
    strings.
    Add RootFrequency as a special type for the Root.frequency
    parameter which is not scaled.
    Add ClockPeriod parameter type.
python/m5/convert.py:
    Be more strict about what's allowed.
    Only accept strings as inputs for these conversion functions.
    If the user wants to accept something else, they need to deal
    with the failure and convert other types on their own.
python/m5/objects/Bus.mpy:
    Use the new ClockPeriod parameter type
python/m5/objects/Root.mpy:
    Can't use integers for frequency anymore
python/m5/smartdict.py:
    rename SmartDict.Proxy to just Variable.  Create a new class
    UndefinedVariable that is returned when the user tries to get
    a variable that is not in the dict.  Undefined variable evaluates
    to false, and will cause an error elsewhere.

--HG--
extra : convert_revision : 1d55246fd1af65106f102396234827d6401ef9ce