dmaengine: pxa: use a dma slave map

In order to remove the specific knowledge of the dma mapping from PXA
drivers, add a default slave map for pxa architectures.

This won't impact MMP architecture, but is aimed only at all PXA boards.

This is the first step, and once all drivers are converted,
pxad_filter_fn() will be made static, and the DMA resources removed from
device.c.

Signed-off-by: Robert Jarzmik <robert.jarzmik@free.fr>
Reported-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Vinod Koul <vkoul@kernel.org>