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

Signed-off-by: Matus Ujhelyi <matus.ujhelyi@streamunlimited.com>
Cc: Grant Likely <grant.likely@secretlab.ca>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

Documentation/devicetree/bindings/video/backlight/tps65217-backlight.txt

+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 */
+		};
+	};