dt-bindings: iio: adc: tlc4541 - recover accidentally dropped binding doc

JC: Seems that I messed up applying the original driver patches, and
this file never actually made it into the tree.

I have picked up original Ack and Sign-off so as to record the
history.
Signed-off-by: Phil Reid <preid@electromag.com.au>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Acked-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20200809111753.156236-13-jic23@kernel.org

Documentation/devicetree/bindings/iio/adc/ti-tlc4541.txt

+* Texas Instruments' TLC4541
+
+Required properties:
+ - compatible: Should be one of
+	* "ti,tlc4541"
+	* "ti,tlc3541"
+	- reg: SPI chip select number for the device
+ - vref-supply: The regulator supply for ADC reference voltage
+ - spi-max-frequency: Max SPI frequency to use (<= 200000)
+
+Example:
+adc@0 {
+	compatible = "ti,tlc4541";
+	reg = <0>;
+	vref-supply = <&vdd_supply>;
+	spi-max-frequency = <200000>;
+};