Documentation: add missing documentation for tps65217-bl device tree binding

TPS65217 family of regulators

The TPS65217 chip contains a boost converter and current sinks which can be

used to drive LEDs for use as backlights.

Required properties:

- compatible: "ti,tps65217"

- reg: I2C slave address

- backlight: node for specifying WLED1 and WLED2 lines in TPS65217

- isel: selection bit, valid values: 1 for ISEL1 (low-level) and 2 for ISEL2 (high-level)

- fdim: PWM dimming frequency, valid values: 100, 200, 500, 1000

Each regulator is defined using the standard binding for regulators.

Example:

	tps: tps@24 {

		reg = <0x24>;

		compatible = "ti,tps65217";

		backlight {

			isel = <1>;  /* 1 - ISET1, 2 ISET2 */

			fdim = <100>; /* TPS65217_BL_FDIM_100HZ */

		};

	};