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

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>;

	};