spi/pxa2xx: enable DMA on newer Intel LPSS silicon

There is an additional bit in the Intel LPSS SPI private registers that
needs to be set in order to be able to use DMA with the SPI controller.
Enable this as well.

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