spi: pxa2xx: Only claim CS GPIOs when the slave device is created

Avoid hogging chip select GPIOs just because they are listed for the
master. They might be mulitplexed and, if no slave device is attached,
used for different purposes. Moreover, this strategy avoids having to
allocate a cs_gpiods structure.

Tested on the IOT2000 where the second SPI bus is connected to an
Arduino-compatible connector and multiplexed between SPI, GPIO and PWM
usage.

Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
Signed-off-by: Mark Brown <broonie@kernel.org>