spi: armada-3700: Add documentation for the Armada 3700 SPI Controller

This adds the devicetree bindings documentation for the SPI controller
present in the Marvell Armada 3700 SoCs.

Signed-off-by: Romain Perier <romain.perier@free-electrons.com>
Tested-by: Gregory CLEMENT <gregory.clement@free-electrons.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Mark Brown <broonie@kernel.org>