spi/pxa2xx: allow building on a 64-bit kernel

We are going to use it on 64-bit kernel on Intel Lynxpoint so make sure we
can build it into such kernel.

Signed-off-by: Mika Westerberg <mika.westerberg@linux.intel.com>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>