devicetree: Add Trusted Computing Group to vendor-prefix.txt

Add missing vendor to vendor-prefix.txt.

Trusted Computing Group design common specifications for
TPM (Trusted Platform Module) vendors.
TCG designates a TPM answering to a public specification.

This commit is based on the initial work by Peter Huewe.

Cc: devicetree@vger.kernel.org
Signed-off-by: Christophe Ricard <christophe-h.ricard@st.com>
Acked-by: Rob Herring <robh@kernel.org>
Acked-by: Jarkko Sakkinen <jarkko.sakkinen@linux.intel.com>
Tested-by: Stefan Berger <stefanb@linux.vnet.ibm.com>
Reviewed-by: Stefan Berger <stefanb@linux.vnet.ibm.com>
Signed-off-by: Jarkko Sakkinen <jarkko.sakkinen@linux.intel.com>