amd-xgbe: Performance enhancements

This patch provides some general performance enhancements for the
driver:
  - Modify the default coalescing settings (reduce usec, increase frames)
  - Change the AXI burst length to 256 bytes (default was 16 bytes which
    was smaller than a cache line)
  - Change the AXI cache settings to write-back/write-allocate which
    allocate cache entries for received packets during the DMA since the
    packet will be processed soon afterwards
  - Combine ioread/iowrite when disabling both the Tx and Rx interrupts
  - Change to processing the Tx/Rx channels in pairs
  - Only recycle the Rx descriptors when a threshold of dirty descriptors
    is reached

Signed-off-by: Tom Lendacky <thomas.lendacky@amd.com>
Signed-off-by: David S. Miller <davem@davemloft.net>