dt-bindings: Add RISC-V vendor prefix

RISC-V systems use device tree to specify the memory layout of the
system.  This patch reserves the "riscv" vendor prefix, which will be
used for devices that are specified by the various RISC-V ISA
specifications.
Signed-off-by: NPalmer Dabbelt <palmer@dabbelt.com>
Signed-off-by: NRob Herring <robh@kernel.org>

Documentation/devicetree/bindings/vendor-prefixes.txt

@@ -270,6 +270,7 @@ renesas	Renesas Electronics Corporation
 richtek	Richtek Technology Corporation
 ricoh	Ricoh Co. Ltd.
 rikomagic	Rikomagic Tech Corp. Ltd
+riscv	RISC-V Foundation
 rockchip	Fuzhou Rockchip Electronics Co., Ltd
 rohm	ROHM Semiconductor Co., Ltd
 roofull	Shenzhen Roofull Technology Co, Ltd