gpio: SPEAr: add spi chipselect control driver

SPEAr platform provides a provision to control chipselects of ARM PL022
Prime Cell spi controller through its system registers, which otherwise
remains under PL022 control which some protocols do not want.

This commit intends to provide the spi chipselect control in software over
gpiolib interface. spi chip drivers can use the exported gpiolib interface to
define their chipselect through DT or platform data.

Cc: Grant Likely <grant.likely@secretlab.ca>
Signed-off-by: Shiraz Hashim <shiraz.hashim@st.com>
Reviewed-by: Vipin Kumar <vipin.kumar@st.com>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>