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>