phy: Add support for the NC-SI protocol

This introduces support for the NC-SI protocol, modelled as a phy driver
for other ethernet drivers to consume.

NC-SI (Network Controller Sideband Interface) is a protocol to manage a
sideband connection to a proper network interface, for example a BMC
(Baseboard Management Controller) sharing the NIC of the host system.
Probing and configuration occurs by communicating with the "remote" NIC
via NC-SI control frames (Ethernet header 0x88f8).

This implementation is roughly based on the upstream Linux
implementation[0], with a reduced feature set and an emphasis on getting
a link up as fast as possible rather than probing the full possible
topology of the bus.
The current phy model relies on the network being "up", sending NC-SI
command frames via net_send_packet() and receiving them from the
net_loop() loop (added in a following patch).

The ncsi-pkt.h header[1] is copied from the Linux kernel for consistent
field definitions.

[0]: https://github.com/torvalds/linux/tree/master/net/ncsi
[1]: https://github.com/torvalds/linux/blob/master/net/ncsi/ncsi-pkt.h



Signed-off-by: Samuel Mendoza-Jonas <sam@mendozajonas.com>
Reviewed-by: Joel Stanley <joel@jms.id.au>
Acked-by: Joe Hershberger <joe.hershberger@ni.com>