spi: qup: Add device tree bindings information

The Qualcomm Universal Peripheral (QUP) core is an
AHB slave that provides a common data path (an output
FIFO and an input FIFO) for serial peripheral interface
(SPI) mini-core.

Signed-off-by: Ivan T. Ivanov <iivanov@mm-sol.com>
Signed-off-by: Mark Brown <broonie@linaro.org>