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>

Documentation/devicetree/bindings/vendor-prefixes.txt

@@ -256,6 +256,7 @@ synology	Synology, Inc.
 SUNW	Sun Microsystems, Inc
 tbs	TBS Technologies
 tcl	Toby Churchill Ltd.
+tcg	Trusted Computing Group
 technologic	Technologic Systems
 thine	THine Electronics, Inc.
 ti	Texas Instruments