net: axienet: make use of axistream-connected attribute optional

Currently the axienet driver requires the use of a second devicetree
node, referenced by an axistream-connected attribute on the Ethernet
device node, which contains the resources for the AXI DMA block used by the
device. This setup is problematic for a use case we have where the Ethernet
and DMA cores are behind a PCIe to AXI bridge and the memory resources for
the nodes are injected into the platform devices using the multifunction
device subsystem - it's not easily possible for the driver to obtain the
platform-level resources from the linked device.

In order to simplify that usage model, and simplify the overall use of
this driver in general, allow for all of the resources to be kept on one
node where the resources are retrieved using platform device APIs rather
than device-tree-specific ones. The previous usage setup is still
supported if the axistream-connected attribute is specified.

Signed-off-by: Robert Hancock <hancock@sedsystems.ca>
Signed-off-by: David S. Miller <davem@davemloft.net>