dt-bindings: pinctrl: Document canaan,k210-fpioa bindings

Document the device tree bindings for the Canaan Kendryte K210 SoC
Fully Programmable IO Array (FPIOA) pinctrl driver in
Documentation/devicetree/bindings/pinctrl/canaan,k210-fpioa.yaml. The
new header file include/dt-bindings/pinctrl/k210-fpioa.h is added to
define all 256 possible pin functions of the SoC IO pins, as well as
macros simplifying the definition of pin functions in a device tree.

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>