dt-bindings: reset: Document canaan,k210-rst bindings

Document the device tree bindings for the Canaan Kendryte K210 SoC
reset controller driver in
Documentation/devicetree/bindings/reset/canaan,k210-rst.yaml. The header
file include/dt-bindings/reset/k210-rst.h is added to define all
possible reset lines of the SoC.

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>