dt-bindings: update sifive clint compatible string

Add the "canaan,k210-clint" compatible string to the Sifive clint
bindings to indicate the use of the "sifive,clint0" IP block in the
Canaan Kendryte K210 SoC. The description of the compatible string
property is also updated to reflect this addition.

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>