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

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>