axgbe: driver changes for netmap support

AMD 10GbE hardware is designed to have two buffers per receive descriptor to
support split header feature. For this purpose, the driver was designed to use
2 iflib freelists per receive queue. So, that buffers from 2 freelists are used
to refill an entry in the receive descriptor. The current design holds good
with regular data traffic.

But, when netmap comes into play, the current design will not fit in. The
current netmap interfaces and netmap implementation in iflib doesn't seem
to accomodate the design of 2 freelists per receive queue. So, exercising
Netmap capability with inbuilt tools like bridge, pkt-gen doesn't work with
the 2 freelists driver design.

So, the driver design is changed to accomodate the current netmap interfaces
and netmap implementation in iflib by using single freelist per receive queue
approach when Netmap capability is exercised without disturbing the current
2 freelists approach.
The dev.ax.sph_enable tunable can be set to 0 to configure the single
free list mode.

Thanks to Stephan Dewt for his Initial set of code changes for the stated
problem.

Submitted by: rajesh1.kumar_amd.com
Approved by: vmaffione
MFC after: 3 weeks
Differential Revision: https://reviews.freebsd.org/D27797