spi: pxa2xx: Introduce DMA burst size support

Some masters may have different DMA burst size than hard coded default.
In such case respect the value given by DMA burst size provided via
platform data.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Tested-by: Jarkko Nikula <jarkko.nikula@linux.intel.com>
Reviewed-by: Jarkko Nikula <jarkko.nikula@linux.intel.com>
Signed-off-by: Mark Brown <broonie@kernel.org>