dt-bindings: tpm: Document Microsoft fTPM bindings

A driver for Microsoft's firmware-based Trusted Platform Module (fTPM)
was merged with commit 09e57483

 ("tpm/tpm_ftpm_tee: A driver for
firmware TPM running inside TEE"), but its devicetree bindings were not.

This is the only remaining undocumented compatible string for a TPM,
so add a DT schema based on the patch linked below.

Link: https://lore.kernel.org/all/20190409184958.7476-2-sashal@kernel.org/
Signed-off-by: Lukas Wunner <lukas@wunner.de>
Reviewed-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/99523409eb5aec9276055ff358ae6f2ceb10be6d.1702806810.git.lukas@wunner.de
Signed-off-by: Rob Herring <robh@kernel.org>