net/ethoc: implement MDIO bus and support phylib

Implement MDIO bus read/write functions, initialize the bus and scan for
the PHY when phylib is enabled. Limit PHY speeds to 10/100 Mbps.

Cc: Michal Simek <monstr@monstr.eu>
Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>
Acked-by: Joe Hershberger <joe.hershberger@ni.com>