spi: Qualcomm Quad SPI(QSPI) documentation

Bindings for Qualcomm Quad SPI used on SoCs such as sdm845.

Signed-off-by: Girish Mahadevan <girishm@codeaurora.org>
Signed-off-by: Ryan Case <ryandcase@chromium.org>
Reviewed-by: Douglas Anderson <dianders@chromium.org>
Reviewed-by: Rob Herring <robh@kernel.org>
Reviewed-by: Stephen Boyd <swboyd@chromium.org>
Signed-off-by: Mark Brown <broonie@kernel.org>

Documentation/devicetree/bindings/spi/qcom,spi-qcom-qspi.txt

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+Qualcomm Quad Serial Peripheral Interface (QSPI)
+
+The QSPI controller allows SPI protocol communication in single, dual, or quad
+wire transmission modes for read/write access to slaves such as NOR flash.
+
+Required properties:
+- compatible:	An SoC specific identifier followed by "qcom,qspi-v1", such as
+		"qcom,sdm845-qspi", "qcom,qspi-v1"
+- reg:		Should contain the base register location and length.
+- interrupts:	Interrupt number used by the controller.
+- clocks:	Should contain the core and AHB clock.
+- clock-names:	Should be "core" for core clock and "iface" for AHB clock.
+
+SPI slave nodes must be children of the SPI master node and can contain
+properties described in Documentation/devicetree/bindings/spi/spi-bus.txt
+
+Example:
+
+	qspi: spi@88df000 {
+		compatible = "qcom,sdm845-qspi", "qcom,qspi-v1";
+		reg = <0x88df000 0x600>;
+		#address-cells = <1>;
+		#size-cells = <0>;
+		interrupts = <GIC_SPI 82 IRQ_TYPE_LEVEL_HIGH>;
+		clock-names = "iface", "core";
+		clocks = <&gcc GCC_QSPI_CNOC_PERIPH_AHB_CLK>,
+			 <&gcc GCC_QSPI_CORE_CLK>;
+
+		flash@0 {
+			compatible = "jedec,spi-nor";
+			reg = <0>;
+			spi-max-frequency = <25000000>;
+			spi-tx-bus-width = <2>;
+			spi-rx-bus-width = <2>;
+		};
+	};