spi/bcm63xx-hsspi: document bcm63xx HS SPI devicetree bindings

Add documentation for the bindings of the high speed SPI controller found
on newer bcm63xx SoCs.
Signed-off-by: Jonas Gorski <jonas.gorski@gmail.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Mark Brown <broonie@kernel.org>

Documentation/devicetree/bindings/spi/spi-bcm63xx-hsspi.txt

+Binding for Broadcom BCM6328 High Speed SPI controller
+
+Required properties:
+- compatible: must contain of "brcm,bcm6328-hsspi".
+- reg: Base address and size of the controllers memory area.
+- interrupts: Interrupt for the SPI block.
+- clocks: phandles of the SPI clock and the PLL clock.
+- clock-names: must be "hsspi", "pll".
+- #address-cells: <1>, as required by generic SPI binding.
+- #size-cells: <0>, also as required by generic SPI binding.
+
+Optional properties:
+- num-cs: some controllers have less than 8 cs signals. Defaults to 8
+  if absent.
+
+Child nodes as per the generic SPI binding.
+
+Example:
+
+	spi@10001000 {
+		compatible = "brcm,bcm6328-hsspi";
+		reg = <0x10001000 0x600>;
+
+		interrupts = <29>;
+
+		clocks = <&clkctl 9>, <&hsspi_pll>;
+		clock-names = "hsspi", "pll";
+
+		num-cs = <2>;
+
+		#address-cells = <1>;
+		#size-cells = <0>;
+	};