dt-binding: clock: Document canaan,k210-clk bindings

Document the device tree bindings of the Canaan Kendryte K210 SoC clock
driver in Documentation/devicetree/bindings/clock/canaan,k210-clk.yaml.
The header file include/dt-bindings/clock/k210-clk.h is modified to
include the complete list of IDs for all clocks of the SoC.

Signed-off-by: Damien Le Moal <damien.lemoal@wdc.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20201220085725.19545-3-damien.lemoal@wdc.com
Signed-off-by: Stephen Boyd <sboyd@kernel.org>