dt-bindings: Add "sifive" vendor prefix

This adds a vendor prefix "sifive" for SiFive, Inc.
We make chips.

Signed-off-by: Wesley W. Terpstra <wesley@sifive.com>
Signed-off-by: Rob Herring <robh@kernel.org>

Documentation/devicetree/bindings/vendor-prefixes.txt

@@ -318,6 +318,7 @@ sgx	SGX Sensortech
 sharp	Sharp Corporation
 shimafuji	Shimafuji Electric, Inc.
 si-en	Si-En Technology Ltd.
+sifive	SiFive, Inc.
 sigma	Sigma Designs, Inc.
 sii	Seiko Instruments, Inc.
 sil	Silicon Image