devicetree: add Sigma Designs vendor prefix

Add the "sigma" vendor prefix for Sigma Designs, Inc.
Signed-off-by: NMans Rullgard <mans@mansr.com>
Signed-off-by: NRob Herring <robh@kernel.org>

Documentation/devicetree/bindings/vendor-prefixes.txt

@@ -193,6 +193,7 @@ schindler	Schindler
 seagate	Seagate Technology PLC
 semtech	Semtech Corporation
 sharp	Sharp Corporation
+sigma	Sigma Designs, Inc.
 sil	Silicon Image
 silabs	Silicon Laboratories
 siliconmitus	Silicon Mitus, Inc.