docs: Extend trusted keys documentation for TPM 2.0

Extend the documentation for trusted keys with documentation for how to
set up a key for a TPM 2.0 so it can be used with a TPM 2.0 as well.

Signed-off-by: Stefan Berger <stefanb@linux.ibm.com>
Reviewed-by: Mimi Zohar <zohar@linux.ibm.com>
Reviewed-by: Dave Jiang <dave.jiang@intel.com>
Acked-by: Dan Williams <dan.j.williams@intel.com>
Acked-by: Jerry Snitselaar <jsnitsel@redhat.com>
Signed-off-by: Mimi Zohar <zohar@linux.ibm.com>