dt-bindings: update sifive uart compatible string

Add the compatible string "canaan,k210-uarths" to the sifive uart
bindings to indicate the use of this IP block in the Canaan Kendryte
K210 SoC.

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