Adding L2Cache Model - new sparta Unit (#114)

-> L2Cache can accept multiple requests from DCache and IL1.
-> Cache functional model used is SimpleDL1.
-> Configurable cache hit latency. 
-> Misses are forwarded to BIU and kept are tracked in the local buffer.
-> Miss to the pending cacheline is not sent out to BIU.
-> Backpressure handling for BIU credits.
-> make regress clean.

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Signed-off-by: Kunal-Buch <145377279+Kunal-Buch@users.noreply.github.com>
Co-authored-by: Knute <56927209+klingaard@users.noreply.github.com>