[POWERPC] Add 'mdio' to bus scan id list for platforms with QE UEC

Add 'mdio' to bus scan id list for platforms with QE UEC
as a consequence of converting UEC mdio driver to an
of_platform driver in the ucc_geth phylib conversion patch.

Signed-off-by: Kim Phillips <kim.phillips@freescale.com>
Signed-off-by: Paul Mackerras <paulus@samba.org>