mfd: devicetree: Add bindings for Atmel Flexcom

This patch documents the DT bindings for the Atmel Flexcom which will be
introduced by sama5d2x SoCs. These bindings will be used by the actual
Flexcom driver to be sent in another patch.

Signed-off-by: Cyrille Pitchen <[e-mail]>
Acked-by: Boris Brezillon <[e-mail]>
Acked-by: Alexandre Belloni <[e-mail]>
Acked-by: Nicolas Ferre <[e-mail]>
Signed-off-by: Lee Jones <[e-mail]>

Documentation/devicetree/bindings/mfd/atmel-flexcom.txt

···
+* Device tree bindings for Atmel Flexcom (Flexible Serial Communication Unit)
+
+The Atmel Flexcom is just a wrapper which embeds a SPI controller, an I2C
+controller and an USART. Only one function can be used at a time and is chosen
+at boot time according to the device tree.
+
+Required properties:
+- compatible:		Should be "atmel,sama5d2-flexcom"
+- reg:			Should be the offset/length value for Flexcom dedicated
+			I/O registers (without USART, TWI or SPI registers).
+- clocks:		Should be the Flexcom peripheral clock from PMC.
+- #address-cells:	Should be <1>
+- #size-cells:		Should be <1>
+- ranges:		Should be one range for the full I/O register region
+			(including USART, TWI and SPI registers).
+- atmel,flexcom-mode:	Should be one of the following values:
+			- <1> for USART
+			- <2> for SPI
+			- <3> for I2C
+
+Required child:
+A single available child device of type matching the "atmel,flexcom-mode"
+property.
+
+The phandle provided by the clocks property of the child is the same as one for
+the Flexcom parent.
+
+For other properties, please refer to the documentations of the respective
+device:
+- ../serial/atmel-usart.txt
+- ../spi/spi_atmel.txt
+- ../i2c/i2c-at91.txt
+
+Example:
+
+flexcom@f8034000 {
+	compatible = "atmel,sama5d2-flexcom";
+	reg = <0xf8034000 0x200>;
+	clocks = <&flx0_clk>;
+	#address-cells = <1>;
+	#size-cells = <1>;
+	ranges = <0x0 0xf8034000 0x800>;
+	atmel,flexcom-mode = <2>;
+
+	spi@400 {
+		compatible = "atmel,at91rm9200-spi";
+		reg = <0x400 0x200>;
+		interrupts = <19 IRQ_TYPE_LEVEL_HIGH 7>;
+		pinctrl-names = "default";
+		pinctrl-0 = <&pinctrl_flx0_default>;
+		#address-cells = <1>;
+		#size-cells = <0>;
+		clocks = <&flx0_clk>;
+		clock-names = "spi_clk";
+		atmel,fifo-size = <32>;
+
+		mtd_dataflash@0 {
+			compatible = "atmel,at25f512b";
+			reg = <0>;
+			spi-max-frequency = <20000000>;
+		};
+	};
+};