dmaengine: xdmac: Add function to align width

The code has some logic to compute the burst width according to the alignment
of the address we're using.

Move that in a function of its own to reduce code duplication.

Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Acked-by: Ludovic Desroches <ludovic.desroches@atmel.com>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>