spi: dw: add support for gpio controlled chip select

Also, use this opportunity to let spi_chip_sel() handle chip-select
deactivation as well.

Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Signed-off-by: Mark Brown <broonie@linaro.org>

diff --cc drivers/spi/spi-dw-mmio.c
Simple merge

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

diff --cc drivers/spi/spi-dw.h
Simple merge