of/mdio: Add support function for Ethernet fixed-link property

Fixed-link support is broken for the ucc_eth, gianfar, and fs_enet
device drivers.  The "OF MDIO rework" patches removed most of the
support. Instead of re-adding fixed-link stuff to the drivers, this
patch adds a support function for parsing the fixed-link property
and obtaining a dummy phy to match.

Note: the dummy phy handling in arch/powerpc is a bit of a hack and
needs to be reworked.  This function is being added now to solve the
regression in the Ethernet drivers, but it should be considered a
temporary measure until the fixed link handling can be reworked.

Signed-off-by: Anton Vorontsov <avorontsov@ru.mvista.com>
Signed-off-by: Grant Likely <grant.likely@secretlab.ca>
Signed-off-by: David S. Miller <davem@davemloft.net>