Multiplier: Make MultiplierIO clone-able

Trying to resolve warning:

[deprecated] class freechips.rocketchip.rocket.MultiplierIO (1 calls): Unable to automatically infer cloneType on class freechips.rocketchip.rocket.MultiplierIO: constructor has parameters (dataBits, tagBits) that are not both immutable and accessible. Either make all parameters immutable and accessible (vals) so cloneType can be inferred, or define a custom cloneType method.
[warn] There were 1 deprecated function(s) used. These may stop compiling in a future release - you are encouraged to fix these issues.
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