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: NHans-Christian Egtvedt <hans-christian.egtvedt@atmel.com>
Signed-off-by: NJamie Iles <jamie.iles@picochip.com>
Signed-off-by: NDan Williams <dan.j.williams@intel.com>