spi: add binding for clps711x SPI

This documents the binding used by Alexander Shiyan's DT support for
the clps711x SPI controller.

I've left the file name to match the ARM platform port name "clps711x"
for consistency with the other bindings, even though the compatible
string refers to the later ep7309 chip.

Linux no longer supports the old clps711x and ep72xx product lines,
but we still use the name. The entire family is now discontinued
by the manufacturer.
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Mark Brown <broonie@kernel.org>

Documentation/devicetree/bindings/spi/spi-clps711x.txt

+Serial Peripheral Interface on Cirrus Logic CL-PS71xx, EP72xx, EP73xx
+
+Required properties
+- #address-cells: must be <1>
+- #size-cells: must be <0>
+- compatible: should include "cirrus,ep7209-spi"
+- reg: Address and length of one register range
+- interrupts: one interrupt line
+- clocks: One entry, refers to the SPI bus clock
+- cs-gpios: Specifies the gpio pins to be used for chipselects.
+	    See: Documentation/devicetree/bindings/spi/spi-bus.txt
+
+An additional register is present in the system controller,
+which is assumed to be in the same device tree, with and marked
+as compatible with "cirrus,ep7209-syscon3".
+
+Example:
+
+spi@80000500 {
+	#address-cells = <1>;
+	#size-cells = <0>;
+	compatible = "cirrus,ep7209-spi";
+	reg = <0x80000500 0x4>;
+	interrupts = <15>;
+	clocks = <&clks CLPS711X_CLK_SPI>;
+	status = "disabled";
+};
+
+syscon3: syscon@80002200 {
+	compatible = "cirrus,ep7209-syscon3", "syscon";
+	reg = <0x80002200 0x40>;
+};
+