ARM PL022 SPI controller

Required properties:
- compatible : "arm,pl022", "arm,primecell"
- reg : Offset and length of the register set for the device
- interrupts : Should contain SPI controller interrupt

Optional properties:
- cs-gpios : should specify GPIOs used for chipselects.
  The gpios will be referred to as reg = <index> in the SPI child nodes.
  If unspecified, a single SPI device without a chip select can be used.


Add ARM pl022 spi controller binding doc
devicetree: Add ARM pl022 spi controller binding doc

Add binding documentation for ARM's Primecell PL022 SPI controller.

Signed-off-by: Rob Herring <aa00819e304141d872f2e4b1c4be422ea3abbfe1@calxeda.com>
