dmaengine: at_xdmac: allow muliple dwidths when doing slave transfers

When using FIFO, we need to support differents data width in a single
transfer. For example, serial device which usually uses 1-byte data
width will use 4-bytes data width when using the FIFO. If the transfer
size is not aligned on 4-bytes then the end of the transfer will be
performed with 1-byte data-width. For that reason,
at_xdmac_prep_slave_sg() now builds linked list descriptors using view 2
instead of view 1 so each of them can update the DWIDTH field into the
Channel Configuration Register.

Signed-off-by: Cyrille Pitchen <cyrille.pitchen@atmel.com>
Signed-off-by: Ludovic Desroches <ludovic.desroches@atmel.com>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>