dt-binding: spi: add bindings for spi-ar934x

Add binding documentation for SPI controller in Qualcomm Atheros
AR934x/QCA95xx SoCs.

Signed-off-by: Chuanhong Guo <gch981213@gmail.com>
Link: https://lore.kernel.org/r/20200210034152.49063-3-gch981213@gmail.com


Signed-off-by: Mark Brown <broonie@kernel.org>