spi: pic32-sqi: add binding document for PIC32 Quad-SPI driver.

Document Device tree bindings for the quad SPI peripheral
found on Microchip PIC32 class devices.
Signed-off-by: Purna Chandra Mandal <purna.mandal@microchip.com>
Acked-by: Rob Herring <rob@kernel.org>

Cc: Kumar Gala <galak@codeaurora.org>
Cc: Ian Campbell <ijc+devicetree@hellion.org.uk>
Cc: Rob Herring <robh+dt@kernel.org>
Cc: Pawel Moll <pawel.moll@arm.com>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Mark Brown <broonie@kernel.org>
Signed-off-by: Mark Brown <broonie@kernel.org>

Documentation/devicetree/bindings/spi/sqi-pic32.txt

+Microchip PIC32 Quad SPI controller
+-----------------------------------
+Required properties:
+- compatible: Should be "microchip,pic32mzda-sqi".
+- reg: Address and length of SQI controller register space.
+- interrupts: Should contain SQI interrupt.
+- clocks: Should contain phandle of two clocks in sequence, one that drives
+          clock on SPI bus and other that drives SQI controller.
+- clock-names: Should be "spi_ck" and "reg_ck" in order.
+
+Example:
+	sqi1: spi@1f8e2000 {
+		compatible = "microchip,pic32mzda-sqi";
+		reg = <0x1f8e2000 0x200>;
+		clocks = <&rootclk REF2CLK>, <&rootclk PB5CLK>;
+		clock-names = "spi_ck", "reg_ck";
+		interrupts = <169 IRQ_TYPE_LEVEL_HIGH>;
+	};