net: mvpp2: replace TX coalescing interrupts with hrtimer

The PP2 controller is capable of per-CPU TX processing, which means there are
per-CPU banked register sets and queues. Current version of the driver supports
TX packet coalescing - once on given CPU sent packets amount reaches a threshold
value, an IRQ occurs. However, there is a single interrupt line responsible for
CPU0/1 TX and RX events (the latter is not per-CPU, the hardware does not
support RSS).

When the top-half executes the interrupt cause is not known. This is why in
NAPI poll function, along with RX processing, IRQ cause register on both
CPU's is accessed in order to determine on which of them the TX coalescing
threshold might have been reached. Thus the egress processing and releasing the
buffers is able to take place on the corresponding CPU. Hitherto approach lead
to an illegal usage of on_each_cpu function in softirq context.

The problem is solved by resigning from TX coalescing interrupts and separating
egress finalization from NAPI processing. For that purpose a method of using
hrtimer is introduced. In main transmit function (mvpp2_tx) buffers are released
once a software coalescing threshold is reached. In case not all the data is
processed a timer is set on this CPU - in its interrupt context a tasklet is
scheduled in which all queues are processed. At once only one timer per-CPU can
be running, which is controlled by a dedicated flag.

This commit removes TX processing from NAPI polling function, disables hardware
coalescing and enables hrtimer with tasklet, using new per-CPU port structure
(mvpp2_port_pcpu).

Signed-off-by: Marcin Wojtas <mw@semihalf.com>
Signed-off-by: David S. Miller <davem@davemloft.net>