dt-binding: mfd: Document canaan,k210-sysctl bindings

Document the device tree bindings of the Canaan Kendryte K210 SoC
system controller driver in
Documentation/devicetree/bindings/mfd/canaan,k210-sysctl.yaml.

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