spi: spi-mpc512x-psc: add support for gpio chip selects

Currently the driver only uses one internal chip select.
Add support for gpio chip selects configured by cs-gpios
DT binding.

Signed-off-by: Anatolij Gustschin <agust@denx.de>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>