Allwinner Gigabit EMAC performance improvements.

• Support DEVICE_POLLING
• Increase TX descriptors to 1024
• Add support for passing a chain of mbufs to if_input, reducing the number of calls to mtx_unlock/mtx_lock under load.
• Remove duplicate byteswap when setting TX_INT_CTL in TX descriptor.
• Set undocumented "TX_NEXT_FRAME" bit in TX control 1 register. According to the A83T BSP, setting this bit allows the DMA engine to operate on a packet while receiving another.

Tested on A83T (1000Mbps PHY) and H3 (100Mbps PHY).

Reviewed by: manu
Differential Revision: https://reviews.freebsd.org/D7031