doc: add freebsd ports

doc: add freebsd ports

Since the DPDK is now part of the BSD ports collection, we should
recommend installing from ports as the best way to get it up and
running.
In order to achieve this, while still keeping the document readable, the
chapter on system requirements has been moved to instead be a section
within the chapter on compiling the DPDK outside of the ports
collection. This move is necessary, since it covered a lot of detail on
installing other ports required to build DPDK. These steps are not
needed when installing DPDK itself from ports.

Signed-off-by: Bruce Richardson <bruce.richardson@intel.com>
Acked-by: Bernard Iremonger <bernard.iremonger@intel.com>