spi: spi-ath79: Add device tree support

Set the OF node of the spi controller and use the generic GPIO based
chip select instead of the custom controller data. As the controller
data isn't used by any board just drop it.

Signed-off-by: Alban Bedel <albeu@free.fr>
Signed-off-by: Mark Brown <broonie@kernel.org>

diff --cc arch/mips/include/asm/mach-ath79/ath79_spi_platform.h
Simple merge

diff --cc drivers/spi/spi-ath79.c
Simple merge