tpm2: add tpm2_get_capability(), tpm2_cache_capabilities(), tpm2_capability_pcrs()

tpm2: add tpm2_get_capability(), tpm2_cache_capabilities(), tpm2_capability_pcrs()

This adds a function to query specific capabilities from the TPM. That is then
used in a function to query the allocation of PCRs in the TPM, i.e. which PCR
banks and indexes are available, and caches the PCR allocation when the TPM
context is created.