dt-bindings: update sifive plic compatible string

Add the compatible string "canaan,k210-plic" to the Sifive plic bindings
to indicate the use of the "sifive,plic-1.0.0" IP block in the Canaan
Kendryte K210 SoC. The description is also updated to reflect this
change, that is, that SoCs from other vendors may also use this plic
implementation.

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