net: ethernet: ti: cpsw: add multi queue support

The cpsw h/w supports up to 8 tx and 8 rx channels. This patch adds
multi-queue support to the driver only, shaper configuration will
be added with separate patch series. Default shaper mode, as
before, priority mode, but with corrected priority order, 0 - is
highest priority, 7 - lowest.

The poll function handles all unprocessed channels, till all of
them are free, beginning from hi priority channel.

In dual_emac mode the channels are shared between two network devices,
as it's with single-queue default mode.

The statistic for every channel can be read with:
$ ethtool -S ethX

Signed-off-by: Ivan Khoronzhuk <ivan.khoronzhuk@linaro.org>
Reviewed-by: Mugunthan V N <mugunthanvnm@ti.com>
Signed-off-by: David S. Miller <davem@davemloft.net>