dt-bindings: update MAINTAINERS file

Add a reference to the Canaan K210 system controller driver bindings
file Documentation/devicetree/bindings/mfd/canaan,k210-sysctl.yaml
in the MAINTAINERS file entry for this driver.

Signed-off-by: Damien Le Moal <damien.lemoal@wdc.com>
Signed-off-by: Palmer Dabbelt <palmerdabbelt@google.com>