i2c: sunxi: add P2WI DT bindings documentation

* Allwinner P2WI (Push/Pull 2 Wire Interface) controller

Required properties :

 - reg             : Offset and length of the register set for the device.

 - compatible      : Should one of the following:

                     - "allwinner,sun6i-a31-p2wi"

 - interrupts      : The interrupt line connected to the P2WI peripheral.

 - clocks          : The gate clk connected to the P2WI peripheral.

 - resets          : The reset line connected to the P2WI peripheral.

Optional properties :

 - clock-frequency : Desired P2WI bus clock frequency in Hz. If not set the

default frequency is 100kHz

A P2WI may contain one child node encoding a P2WI slave device.

Slave device properties:

  Required properties:

   - reg           : the I2C slave address used during the initialization

                     process to switch from I2C to P2WI mode

Example:

	p2wi@01f03400 {

		compatible = "allwinner,sun6i-a31-p2wi";

		reg = <0x01f03400 0x400>;

		interrupts = <0 39 4>;

		clocks = <&apb0_gates 3>;

		clock-frequency = <6000000>;

		resets = <&apb0_rst 3>;

		axp221: pmic@68 {

			compatible = "x-powers,axp221";

			reg = <0x68>;

			/* ... */

		};

	};