dmaengine/dw_dmac: allow src/dst masters to be configured at runtime

Some platforms have flexible mastering capabilities and this needs
to be selected at runtime. If the platform has specified private
data in the form of the dw_dma_slave then fetch the source and
destination masters from here. If this isn't present, default to
the previous of 0 and 1.

v2: cleanup whitespace

Acked-by: Hans-Christian Egtvedt <hans-christian.egtvedt@atmel.com>
Signed-off-by: Jamie Iles <jamie.iles@picochip.com>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>