dt-bindings: qca7000-spi: Rework binding

* Qualcomm QCA7000 (Ethernet over SPI protocol)

* Qualcomm QCA7000

Note: The QCA7000 is useable as a SPI device. In this case it must be defined

as a child of a SPI master in the device tree.

The QCA7000 is a serial-to-powerline bridge with a host interface which could

be configured either as SPI or UART slave. This configuration is done by

the QCA7000 firmware.

(a) Ethernet over SPI

In order to use the QCA7000 as SPI device it must be defined as a child of a

SPI master in the device tree.

Required properties:

- compatible	    : Should be "qca,qca7000"

- reg		    : Should specify the SPI chip select

- interrupts : The first cell should specify the index of the source interrupt

  and the second cell should specify the trigger type as rising edge

- interrupts	    : The first cell should specify the index of the source

		      interrupt and the second cell should specify the trigger

		      type as rising edge

- spi-cpha	    : Must be set

- spi-cpol	    : Must be set

Optional properties:

- interrupt-parent  : Specify the pHandle of the source interrupt

- spi-max-frequency : Maximum frequency of the SPI bus the chip can operate at.

  Numbers smaller than 1000000 or greater than 16000000 are invalid. Missing

  the property will set the SPI frequency to 8000000 Hertz.

- local-mac-address: 6 bytes, MAC address

		      Numbers smaller than 1000000 or greater than 16000000

		      are invalid. Missing the property will set the SPI

		      frequency to 8000000 Hertz.

- local-mac-address : see ./ethernet.txt

- qca,legacy-mode   : Set the SPI data transfer of the QCA7000 to legacy mode.

  In this mode the SPI master must toggle the chip select between each data

  word. In burst mode these gaps aren't necessary, which is faster.

  This setting depends on how the QCA7000 is setup via GPIO pin strapping.

  If the property is missing the driver defaults to burst mode.

		      In this mode the SPI master must toggle the chip select

		      between each data word. In burst mode these gaps aren't

		      necessary, which is faster. This setting depends on how

		      the QCA7000 is setup via GPIO pin strapping. If the

		      property is missing the driver defaults to burst mode.

Example:

SPI Example:

/* Freescale i.MX28 SPI master*/

ssp2: spi@80014000 {