dmaengine: at_hdmac: add slave config operation

This patch introduces DMA_SLAVE_CONFIG to at_hdmac Atmel DMA driver.

It is needed to fix a regression in the use of atmel-mci.c driver on Atmel
AT91 platforms brouth by e2b35f3:
"dmaengine/dw_dmac: Fix dw_dmac user drivers to adapt to slave_config changes"

We remove some parts of the private structure "at_dma_slave" and use the
information provided by "struct dma_slave_config": source/destination
peripheral registers and access width.

AT_DMA_SLAVE_WIDTH_* values used previously are not needed anymore as we
now use the standard ones. Although some conversion functions are needed to
match register expected values.

Some AT91 sub-architecture specific files are slightly touched by this patch
but it cannot be split because it can break compilation.

Signed-off-by: Nicolas Ferre <nicolas.ferre@atmel.com>
Signed-off-by: Vinod Koul <vinod.koul@linux.intel.com>