net: macb: add gmac multi-queue support

net: macb: add gmac multi-queue support

This patch refer to linux kernel commit: d8b763e1e79f
  net/macb: add TX multiqueue support for gem
  by: Cyrille Pitchen

1. macb driver will check the register to find how many queues support for
this chip.

2. Then as we only use queue0 for tx, so we will set up all other queues
use a dummy descriptor, which USED bit is set. So those queues are not used.

Signed-off-by: Josh Wu <josh.wu@atmel.com>
Acked-by: Joe Hershberger <joe.hershberger@ni.com>