pcnet: add pcn driver

* le still working ok with QEmu
* pcn brings support for the following devices in 32-bit mode (instead of 16-bit Lance
compatibility mode), all untested, feedback is welcome.
AMD Am79C971 PCnet-FAST, AMD Am79C972 PCnet-FAST+, AMD Am79C973/Am79C975 PCnet-FAST III,
AMD Am79C976 PCnet-PRO, AMD Am79C978 PCnet-Home, Allied-Telesis LA-PCI
* added PHY nsphy, nsphyter and ukphy, which seem the ones needed for pcn.
* synched miidevs with FreeBSD 9.2