net: mvpp2: introduce queue_vector concept

In preparation to the introduction of TX interrupts and improved RX
queue distribution, this commit introduces the concept of "queue
vector". A queue vector represents a number of RX and/or TX queues,
and an associated NAPI instance and interrupt.

This commit currently only creates a single queue_vector, so there are
no changes in behavior, but it paves the way for additional
queue_vector in the next commits.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: David S. Miller <davem@davemloft.net>