spi: axi: Add bindings documentation for Analog Devices axi-spi-engine

Add the devicetree bindings documentation for the Analog Devices
axi-spi-engine SPI master peripheral. This is a soft-peripheral used in
FPGAs.

The external interfaces of the peripheral are:
	* A memory mapped register map which is used to configure the
	  peripheral.
	* One interrupt.
	* Two clocks, one for the memory mapped register interface and one
	  for the SPI bus.
	* A SPI master interface to which the slave devices are connected.

These interfaces are described by the devicetree bindings accordingly.

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Mark Brown <broonie@kernel.org>