atmel-mci: change use of dma slave interface

Allow the use of another DMA controller driver in atmel-mci sd/mmc driver.
 This adds a generic dma_slave pointer to the mci platform structure where
we can store DMA controller information.  In atmel-mci we use information
provided by this structure to initialize the driver (with new helper
functions that are architecture dependant).

This also adds at32/avr32 chip modifications to cope with this new access
method.

Signed-off-by: Nicolas Ferre <nicolas.ferre@atmel.com>
Cc: Haavard Skinnemoen <hskinnemoen@atmel.com>
Cc: <linux-mmc@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>