spi: pxa2xx: use DMA by default if supported

Currently, even if the PXA2xx SPI master supports DMA, it won't be
enabled unless (i) the slave device is enumerated through ACPI, or
(ii) the slave device is registered with board-specific
controller_data specified.  Even then, there isn't a field in the
controller_data that explicitly enables dma - it just gets enabled
if the master supports it and controller_data is non-NULL.

This means that drivers which register SPI devices on a bus without
awareness of this controller cannot avail of DMA performance gains.

This patch allows DMA transfers to be used if supported.

Signed-off-by: Dan O'Donovan <dan@emutex.com>
Reviewed-by: Jarkko Nikula <jarkko.nikula@linux.intel.com>
Signed-off-by: Mark Brown <broonie@kernel.org>