ath79: add new ar934x spi driver

A new shift mode was introduced since ar934x which has a way better
performance than current bitbang driver and can handle higher spi
clock properly. This commit adds a new driver to make use of this
new feature.
This new driver has chipselect properly configured and we don't need
cs-gpios hack in dts anymore. Remove them.

Signed-off-by: Chuanhong Guo <gch981213@gmail.com>