spi/pl022: Add chip select handling via GPIO

This patch adds the ability for the driver to control the chip select directly.
This enables independence from cs_control callbacks.  Configurable via
platform_data, to be extended as DT in the following patch.

Based on the initial patch by Alexandre Pereira da Silva <aletes.xgr@gmail.com>

Signed-off-by: Roland Stigge <stigge@antcom.de>
Acked-by: Alexandre Pereira da Silva <aletes.xgr@gmail.com>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>