if_arge: fix up TX workaround; add TX/RX requirements for busdma; add stats

The early ethernet MACs (I think AR71xx and AR913x) require that both
TX and RX require 4-byte alignment for all packets.

The later MACs have started relaxing the requirements.

For now, the 1-byte TX and 1-byte RX alignment requirements are only for
the QCA955x SoCs. I'll add in the relaxed requirements as I review the
datasheets and do testing.

• Add a hardware flags field and 1-byte / 4-byte TX/RX alignment.
• .. defaulting to 4-byte TX and 4-byte RX alignment.
• Only enforce the TX alignment fixup if the hardware requires a 4-byte TX alignment. This avoids a call to m_defrag().
• Add counters for various situations for further debugging.
• Set the 1-byte and 4-byte busdma alignment requirement when the tag is created.

This improves the straight bridging performance from 130mbit/sec
to 180mbit/sec, purely by removing the need for TX path bounce buffers.

The main performance issue is the RX alignment requirement and any RX
bounce buffering that's occuring. (In a local test, removing the RX
fixup path and just aligning buffers raises the performance to above
400mbit/sec.

In theory it's a no-op for SoCs before the QCA955x.

Tested:

• QCA9558 SoC in AP135 board, using software bridging between arge0/arge1.