spi: Add binding document for IMG SPFI controller

The Synchronous Peripheral Flash Interface (SPFI) controller found
on IMG SoCs supports single, dual, and (optionally) quad mode SPI
transfers.
Signed-off-by: Andrew Bresticker <abrestic@chromium.org>
Signed-off-by: Mark Brown <broonie@kernel.org>

Documentation/devicetree/bindings/spi/spi-img-spfi.txt

+IMG Synchronous Peripheral Flash Interface (SPFI) controller
+
+Required properties:
+- compatible: Must be "img,spfi".
+- reg: Must contain the base address and length of the SPFI registers.
+- interrupts: Must contain the SPFI interrupt.
+- clocks: Must contain an entry for each entry in clock-names.
+  See ../clock/clock-bindings.txt for details.
+- clock-names: Must include the following entries:
+  - spfi: SPI operating clock
+  - sys: SPI system interface clock
+- dmas: Must contain an entry for each entry in dma-names.
+  See ../dma/dma.txt for details.
+- dma-names: Must include the following entries:
+  - rx
+  - tx
+- #address-cells: Must be 1.
+- #size-cells: Must be 0.
+
+Optional properties:
+- img,supports-quad-mode: Should be set if the interface supports quad mode
+  SPI transfers.
+
+Example:
+
+spi@18100f00 {
+	compatible = "img,spfi";
+	reg = <0x18100f00 0x100>;
+	interrupts = <GIC_SHARED 22 IRQ_TYPE_LEVEL_HIGH>;
+	clocks = <&spi_clk>, <&system_clk>;
+	clock-names = "spfi", "sys";
+	dmas = <&mdc 9 0xffffffff 0>, <&mdc 10 0xffffffff 0>;
+	dma-names = "rx", "tx";
+
+	#address-cells = <1>;
+	#size-cells = <0>;
+};