spi: spi-gpio: store chipselect information in private structure

The spi-gpio driver currently assumes the chipselect gpio number is
stored in ->controller_data of the device's static board information.

In devicetree environments, this information is unavailable and has to
be derived from the DT node.

This patch moves the gpio storage to the controller's private data so
the DT bindings can easily build upon the driver.

Signed-off-by: Daniel Mack <zonque@gmail.com>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>