dt-bindings: add Canaan boards compatible strings

Introduce the file riscv/canaan.yaml to document compatible strings
related to the Canaan Kendryte K210 SoC. The compatible string
"canaan,kendryte-k210" used to indicate the use of this SoC to the
early SoC init code is added. This new file also defines the compatible
strings of all supported boards based on this SoC.

Signed-off-by: Damien Le Moal <damien.lemoal@wdc.com>
Reviewed-by: Atish Patra <atish.patra@wdc.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Palmer Dabbelt <palmerdabbelt@google.com>