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tpm2: Implement a cache for the private exponent D and prime Q

Implement a cache for the private exponent 'D' and prime 'Q' so that we
do not have to recalculate 'Q' and 'D' every time an RSA key is used. For
a cache hit we now use ~34000 cycles and on a cache miss it needs around
130000 cycles. Previously it needed around 100000 cycles to calcuate 'Q'
and 'D'. Assuming that keys will be reused and the cache is big enough
for the number of keys being use (64 entries), it seems well worth it.

This solution is better than extending the OBJECT with 'D' since OBJECT is
kept in the TPM's NVRAM and we would then need more memory to store OBJECTs
there.

Signed-off-by: Stefan Berger <stefanb@linux.ibm.com>

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