Documentation: tpm: add the IBM Virtual TPM device tree binding documentation

Virtual TPM, which is being used on IBM POWER7+ and POWER8 systems running
POWERVM, is currently supported by tpm device driver but lacks the
documentation. This patch adds the missing documentation for the existing
support.

Suggested-by: Jason Gunthorpe <jgunthorpe@obsidianresearch.com>
Signed-off-by: Nayna Jain <nayna@linux.vnet.ibm.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Jarkko Sakkinen <jarkko.sakkinen@linux.intel.com>