xilinx_axidma: Create Proxy object for stream

xilinx_axidma: Create Proxy object for stream

Create a separate child object to proxy the stream slave connection. This is
setup for future work where a second stream slave connection is needed. The
new child object is created at qdev init time and is linked back to the parent
(the ethernet device itself) automatically.

Stream slave masters differentiate which slave connection they are connected to
by linking to the proxy object rather than the parent.

Signed-off-by: Peter Crosthwaite <peter.crosthwaite@xilinx.com>
Signed-off-by: Edgar E. Iglesias <edgar.iglesias@gmail.com>