i2c: sh_mobile: add devicetree documentation

Device tree configuration for Renesas IIC (sh_mobile) driver

Required properties:

- compatible      : "renesas,iic-<soctype>". "renesas,rmobile-iic" as fallback

- reg             : address start and address range size of device

- interrupts      : interrupt of device

- clocks          : clock for device

- #address-cells  : should be <1>

- #size-cells     : should be <0>

Optional properties:

- clock-frequency : frequency of bus clock in Hz. Default 100kHz if unset.

Pinctrl properties might be needed, too. See there.

Example:

	iic0: i2c@e6500000 {

		compatible = "renesas,iic-r8a7790", "renesas,rmobile-iic";

		reg = <0 0xe6500000 0 0x425>;

		interrupts = <0 174 IRQ_TYPE_LEVEL_HIGH>;

		clocks = <&mstp3_clks R8A7790_CLK_IIC0>;

		clock-frequency = <400000>;

		#address-cells = <1>;

		#size-cells = <0>;

	};