tsec: Fix MDIO on devices with eTSEC2

The tsec driver was defining the default MDIO address as
the TSEC_BASE + 0x520, but on eTSEC2 controllers, the first
TSEC's registers are separated from the MDIO registers. Use
the existing MDIO_BASE_ADDR, instead.

Signed-off-by: Andy Fleming <afleming@freescale.com>
Signed-off-by: Kumar Gala <galak@kernel.crashing.org>