dwc_eqos - queue tuning

[idigdoug]

• Try to tune the flow control on the Rx queue. Seemed to have a minor throughput improvement from sending pause packet at about half full.
• Make the flow control tuning more generic so it will work with buffer sizes other than the sizes in the RK3588.
• Try to tune the AXI parameters. Almost nothing I did seemed to have a consistent effect. I think there was a small improvement by enabling address-aligned beats, though it might just be my imagination.