bcm63xx_enet: add support Broadcom BCM6345 Ethernet

This patch adds support for the Broadcom BCM6345 SoC Ethernet. BCM6345
has a slightly different and older DMA engine which requires the
following modifications:

- the width of the DMA channels on BCM6345 is 64 bytes vs 16 bytes,
  which means that the helpers enet_dma{c,s} need to account for this
  channel width and we can no longer use macros

- BCM6345 DMA engine does not have any internal SRAM for transfering
  buffers

- BCM6345 buffer allocation and flow control is not per-channel but
  global (done in RSET_ENETDMA)

- the DMA engine bits are right-shifted by 3 compared to other DMA
  generations

- the DMA enable/interrupt masks are a little different (we need to
  enabled more bits for 6345)

- some register have the same meaning but are offsetted in the ENET_DMAC
  space so a lookup table is required to return the proper offset

The MAC itself is identical and requires no modifications to work.

Signed-off-by: Florian Fainelli <florian@openwrt.org>
Acked-by: Ralf Baechle <ralf@linux-mips.org>
Signed-off-by: David S. Miller <davem@davemloft.net>