Blackfin: bf518f-ezbrd: handle different PHYs dynamically

The original BF518F-EZBRD's have a Micrel KSZ8893 DSA on them, but newer
ones only have a National PHY (which lack a RX Error interrupt line).  So
in the board eth init code, dynamically detect what is hooked up to the MAC
and handle each accordingly.

Signed-off-by: Mike Frysinger <vapier@gentoo.org>