spi/pxa2xx: use GFP_ATOMIC in sg table allocation

pxa2xx_spi_map_dma_buffer() gets called in tasklet context so we can't
sleep when we allocate a new sg table. Use GFP_ATOMIC here instead.

Signed-off-by: Mika Westerberg <mika.westerberg@linux.intel.com>
Signed-off-by: Mark Brown <broonie@linaro.org>
Cc: stable@vger.kernel.org