spi: pxa2xx: Fix build error because of missing header

Kbuild test robot reports:

  drivers/spi/spi-pxa2xx.c: In function ‘setup_cs’:
  drivers/spi/spi-pxa2xx.c:1190:20: error: implicit declaration of function ‘desc_to_gpio’
  ...

Reason for this is the fact that those functions are declared in
linux/gpio/consumer.h which is not included in the driver. Fix this by
including it.

Reported-by: kbuild test robot <fengguang.wu@intel.com>
Signed-off-by: Mika Westerberg <mika.westerberg@linux.intel.com>
Signed-off-by: Mark Brown <broonie@kernel.org>