dt-bindings: Add Canaan vendor prefix

Update Documentation/devicetree/bindings/vendor-prefixes.yaml to
include "canaan" as a vendor prefix for "Canaan Inc.". Canaan is the
vendor of the Kendryte K210 RISC-V SoC.

Signed-off-by: Damien Le Moal <damien.lemoal@wdc.com>
Acked-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20201220085725.19545-2-damien.lemoal@wdc.com
Signed-off-by: Stephen Boyd <sboyd@kernel.org>