tpm: Add TPM CRQ driver implementation

This patch adds a TPM driver for the CRQ interface as used by
the QEMU PAPR implementation.

Also add a Readme that explains the benefits and installation procedure
for the vTPM.

Signed-off-by: Stefan Berger <stefanb@linux.ibm.com>
Signed-off-by: Alexey Kardashevskiy <aik@ozlabs.ru>