dt-bindings: tpm: Convert IBM vTPM bindings to DT schema

Convert the devicetree bindings for the IBM Virtual Trusted Platform
Module to DT schema.  Drop properties which are already documented in
tpm-common.yaml.

Document the "IBM,vtpm20" compatible string introduced by commit
18b3670d

 ("tpm: ibmvtpm: Add support for TPM2").

Signed-off-by: Lukas Wunner <lukas@wunner.de>
Reviewed-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/5c41e045dfe7cc3f27dd41c08c62ed8c4a90d8a4.1702806810.git.lukas@wunner.de
Signed-off-by: Rob Herring <robh@kernel.org>