net: mvpp2: support XPS by mapping TX queues to CPUs

Since the PPv2 controller has multiple TX queues, we can spread traffic
by assining TX queues to CPUs, allowing to use XPS to balance egress
traffic between CPUs.

Suggested-by : Yan Markman <ymarkman@marvell.com>
Signed-off-by: Maxime Chevallier <maxime.chevallier@bootlin.com>
Signed-off-by: David S. Miller <davem@davemloft.net>