[media] si4713: add DT binding documentation

This patch adds the DT bindings documentation for Silicon Labs Si4713 FM
radio transmitter.
Signed-off-by: NSebastian Reichel <sre@kernel.org>
Signed-off-by: NHans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: NMauro Carvalho Chehab <mchehab@osg.samsung.com>

Documentation/devicetree/bindings/media/si4713.txt

+* Silicon Labs FM Radio transmitter
+
+The Silicon Labs Si4713 is an FM radio transmitter with receive power scan
+supporting 76-108 MHz. It includes an RDS encoder and has both, a stereo-analog
+and a digital interface, which supports I2S, left-justified and a custom
+DSP-mode format. It is programmable through an I2C interface.
+
+Required Properties:
+- compatible: Should contain "silabs,si4713"
+- reg: the I2C address of the device
+
+Optional Properties:
+- interrupts-extended: Interrupt specifier for the chips interrupt
+- reset-gpios: GPIO specifier for the chips reset line
+- vdd-supply: phandle for Vdd regulator
+- vio-supply: phandle for Vio regulator
+
+Example:
+
+&i2c2 {
+        fmtx: si4713@63 {
+                compatible = "silabs,si4713";
+                reg = <0x63>;
+
+                interrupts-extended = <&gpio2 21 IRQ_TYPE_EDGE_FALLING>; /* 53 */
+                reset-gpios = <&gpio6 3 GPIO_ACTIVE_HIGH>; /* 163 */
+                vio-supply = <&vio>;
+                vdd-supply = <&vaux1>;
+        };
+};