spi: dw: Convert to use CS GPIO descriptors

This converts the DesignWare (dw) SPI master driver to
use GPIO descriptors for chip select handling.

This driver has a duplicate DT parser in addition to the
one in the core, sets up the line as non-asserted and
relies on the core to drive the GPIOs.

It is a pretty straight-forward conversion.

Cc: Talel Shenhar <talel@amazon.com>
Cc: Simon Goldschmidt <simon.k.r.goldschmidt@gmail.com>
Cc: Alexandre Belloni <alexandre.belloni@bootlin.com>
Cc: Linuxarm <linuxarm@huawei.com>
Tested-by: Jay Fang <f.fangjian@huawei.com>
Reviewed-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Mark Brown <broonie@kernel.org>