enetc: Add adaptive interrupt coalescing

Use the generic dynamic interrupt moderation (dim)
framework to implement adaptive interrupt coalescing
on Rx.  With the per-packet interrupt scheme, a high
interrupt rate has been noted for moderate traffic flows
leading to high CPU utilization.  The 'dim' scheme
implemented by the current patch addresses this issue
improving CPU utilization while using minimal coalescing
time thresholds in order to preserve a good latency.
On the Tx side use an optimal time threshold value by
default.  This value has been optimized for Tx TCP
streams at a rate of around 85kpps on a 1G link,
at which rate half of the Tx ring size (128) gets filled
in 1500 usecs.  Scaling this down to 2.5G links yields
the current value of 600 usecs, which is conservative
and gives good enough results for 1G links too (see
next).

Below are some measurement results for before and after
this patch (and related dependencies) basically, for a
2 ARM Cortex-A72 @1.3Ghz CPUs system (32 KB L1 data cache),
using 60secs log netperf TCP stream tests @ 1Gbit link
(maximum throughput):

1) 1 Rx TCP flow, both Rx and Tx processed by the same NAPI
thread on the same CPU:
	CPU utilization		int rate (ints/sec)
Before:	50%-60% (over 50%)		92k
After:  13%-22%				3.5k-12k
Comment:  Major CPU utilization improvement for a single flow
	  Rx TCP flow (i.e. netperf -t TCP_MAERTS) on a single
	  CPU. Usually settles under 16% for longer tests.

2) 4 Rx TCP flows + 4 Tx TCP flows (+ pings to check the latency):
	Total CPU utilization	Total int rate (ints/sec)
Before:	~80% (spikes to 90%)		~100k
After:   60% (more steady)		  ~4k
Comment:  Important improvement for this load test, while the
	  ping test outcome does not show any notable
	  difference compared to before.

Signed-off-by: Claudiu Manoil <claudiu.manoil@nxp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>