net: mvpp2: add support for TX interrupts and RX queue distribution modes

This commit adds the support for two related features:

 - Support for TX interrupts, with one interrupt for each CPU

 - Support for different RX queue distribution modes
   MVPP2_QDIST_SINGLE_MODE where a single interrupt, shared by all
   CPUs, receives the RX events, and MVPP2_QDIST_MULTI_MODE, where the
   per-CPU interrupts used for TX events are also used for RX events.

Since additional interrupts are needed, an update to the Device Tree
binding is needed. However, backward compatibility is preserved with
the old Device Tree binding, by gracefully degrading to the original
behavior, with only one RX interrupt, and TX completion being handled
by an hrtimer.

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