dma4_init() wrote the system bus mode register with only the burst length
and enhanced address bits, which left the maximum number of outstanding AXI
requests at its reset value of one and did not honour mixed burst. The
receive DMA could not keep up and dropped frames in the FIFO while the CPU
was mostly idle.
Parse the snps,axi-config node of the device tree (the maximum number of
outstanding read and write requests) with the other snps properties, and
program the limits, the hardware fields hold the limit minus one. Set the
mixed burst bit when snps,mixed-burst is present, as the eqos driver does.