spi: pxa2xx: use a module softdep for dw_dmac

With dw_dmac, sometimes the request of a DMA channel fails because
the DMA driver is not ready, so an explicit dependency request
is necessary.
Signed-off-by: Flavio Suligoi <f.suligoi@asem.it>
Signed-off-by: Mark Brown <broonie@kernel.org>

drivers/spi/spi-pxa2xx.c

@@ -1957,3 +1957,5 @@ static void __exit pxa2xx_spi_exit(void)
 	platform_driver_unregister(&driver);
 }
 module_exit(pxa2xx_spi_exit);
+
+MODULE_SOFTDEP("pre: dw_dmac");