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: Palmer Dabbelt <palmer@dabbelt.com>
Signed-off-by: Rob Herring <robh@kernel.org>