MFC of r303230, r303253 and r303420:

Add support for the Microchip/Micrel KSZ9031 Gigabit Ethernet PHY.

Enable the build of micphy as part of generic miibus build, but only for
FDT enabled systems.

The Micrel PHYs reads the optional external delays from DTB.

Tested on uBMC and uFW.

Sponsored by: Rubicon Communications (Netgate)