Documentation: tpm: add the Physical TPM device tree binding documentation

Newly added support of TPM 2.0 eventlog securityfs pseudo files in tpm
device driver consumes device tree bindings representing I2C based
Physical TPM. This patch adds the documentation for corresponding device
tree bindings of I2C based Physical TPM. These bindings are similar to
vtpm device tree bindings being used on IBM Power7+ and Power8 Systems
running PowerVM.

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>