Documentation: afe4403: Add DT bindings for the AFE4403 heart monitor

Add the TI AFE4403 heart monitor DT bindings documentation.
Signed-off-by: Andrew F. Davis <afd@ti.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>

Documentation/devicetree/bindings/iio/health/afe4403.txt

+Texas Instruments AFE4403 Heart rate and Pulse Oximeter
+
+Required properties:
+ - compatible		: Should be "ti,afe4403".
+ - reg			: SPI chip select address of device.
+ - tx-supply		: Regulator supply to transmitting LEDs.
+ - interrupt-parent	: Phandle to he parent interrupt controller.
+ - interrupts		: The interrupt line the device ADC_RDY pin is
+			  connected to. For details refer to,
+			  ../../interrupt-controller/interrupts.txt.
+
+Optional properties:
+ - reset-gpios		: GPIO used to reset the device.
+			  For details refer to, ../../gpio/gpio.txt.
+
+For other required and optional properties of SPI slave nodes
+please refer to ../../spi/spi-bus.txt.
+
+Example:
+
+&spi0 {
+	heart_mon@0 {
+		compatible = "ti,afe4403";
+		reg = <0>;
+		spi-max-frequency = <10000000>;
+
+		tx-supply = <&vbat>;
+
+		interrupt-parent = <&gpio1>;
+		interrupts = <28 IRQ_TYPE_EDGE_RISING>;
+
+		reset-gpios = <&gpio1 16 GPIO_ACTIVE_LOW>;
+	};
+};