wip: add documentation

Documentation/devicetree/bindings/arm/rockchip.yaml

Documentation/devicetree/bindings/arm/rockchip/pmu.yaml

...tation/devicetree/bindings/clock/rockchip,rk3576-cru.yaml

Documentation/devicetree/bindings/i2c/i2c-rk3x.yaml

...entation/devicetree/bindings/iio/adc/rockchip-saradc.yaml

Documentation/devicetree/bindings/mfd/syscon.yaml

Documentation/devicetree/bindings/mmc/rockchip-dw-mshc.yaml

Documentation/devicetree/bindings/nvmem/rockchip,otp.yaml

...ntation/devicetree/bindings/pinctrl/rockchip,pinctrl.yaml

.../devicetree/bindings/power/rockchip,power-controller.yaml

...entation/devicetree/bindings/serial/snps-dw-apb-uart.yaml

Documentation/devicetree/bindings/soc/rockchip/grf.yaml

...entation/devicetree/bindings/timer/rockchip,rk-timer.yaml