mtd: spi-nor: Bindings for Cadence Quad SPI Flash Controller driver

Add binding document for the Cadence QSPI controller.

Signed-off-by: Graham Moore <[e-mail]>
Signed-off-by: Marek Vasut <[e-mail]>
Cc: Alan Tull <[e-mail]>
Cc: Brian Norris <[e-mail]>
Cc: David Woodhouse <[e-mail]>
Cc: Dinh Nguyen <[e-mail]>
Cc: Graham Moore <[e-mail]>
Cc: Vignesh R <[e-mail]>
Cc: Yves Vandervennet <[e-mail]>
Cc: [e-mail]
Acked-by: Rob Herring <[e-mail]>
Signed-off-by: Brian Norris <[e-mail]>

Documentation/devicetree/bindings/mtd/cadence-quadspi.txt

···
+* Cadence Quad SPI controller
+
+Required properties:
+- compatible : Should be "cdns,qspi-nor".
+- reg : Contains two entries, each of which is a tuple consisting of a
+	physical address and length. The first entry is the address and
+	length of the controller register set. The second entry is the
+	address and length of the QSPI Controller data area.
+- interrupts : Unit interrupt specifier for the controller interrupt.
+- clocks : phandle to the Quad SPI clock.
+- cdns,fifo-depth : Size of the data FIFO in words.
+- cdns,fifo-width : Bus width of the data FIFO in bytes.
+- cdns,trigger-address : 32-bit indirect AHB trigger address.
+
+Optional properties:
+- cdns,is-decoded-cs : Flag to indicate whether decoder is used or not.
+
+Optional subnodes:
+Subnodes of the Cadence Quad SPI controller are spi slave nodes with additional
+custom properties:
+- cdns,read-delay : Delay for read capture logic, in clock cycles
+- cdns,tshsl-ns : Delay in nanoseconds for the length that the master
+                  mode chip select outputs are de-asserted between
+		  transactions.
+- cdns,tsd2d-ns : Delay in nanoseconds between one chip select being
+                  de-activated and the activation of another.
+- cdns,tchsh-ns : Delay in nanoseconds between last bit of current
+                  transaction and deasserting the device chip select
+		  (qspi_n_ss_out).
+- cdns,tslch-ns : Delay in nanoseconds between setting qspi_n_ss_out low
+                  and first bit transfer.
+
+Example:
+
+	qspi: spi@ff705000 {
+		compatible = "cdns,qspi-nor";
+		#address-cells = <1>;
+		#size-cells = <0>;
+		reg = <0xff705000 0x1000>,
+		      <0xffa00000 0x1000>;
+		interrupts = <0 151 4>;
+		clocks = <&qspi_clk>;
+		cdns,is-decoded-cs;
+		cdns,fifo-depth = <128>;
+		cdns,fifo-width = <4>;
+		cdns,trigger-address = <0x00000000>;
+
+		flash0: n25q00@0 {
+			...
+			cdns,read-delay = <4>;
+			cdns,tshsl-ns = <50>;
+			cdns,tsd2d-ns = <50>;
+			cdns,tchsh-ns = <4>;
+			cdns,tslch-ns = <4>;
+		};
+	};