spi/atmel: add DT support

Use the newly introduce cs-gpios dt support on atmel.
We do not use the hardware cs as it's wired and has bugs and limitations.
As the controller believes that only active-low devices/systems exists.

Signed-off-by: Jean-Christophe PLAGNIOL-VILLARD <plagnioj@jcrosoft.com>
Acked-by: Nicolas Ferre <nicolas.ferre@atmel.com>
Signed-off-by: Grant Likely <grant.likely@secretlab.ca>