spi: add driver for ar934x spi controller

spi: add driver for ar934x spi controller

This patch adds driver for SPI controller found in Qualcomm Atheros
AR934x/QCA95xx SoCs.
This controller is a superset of the already supported qca,ar7100-spi.
Besides the bit-bang mode in spi-ath79.c, this new controller added
a new "shift register" mode, allowing faster spi operations.

Signed-off-by: Chuanhong Guo <gch981213@gmail.com>
Link: https://lore.kernel.org/r/20200210034152.49063-2-gch981213@gmail.com
Signed-off-by: Mark Brown <broonie@kernel.org>