net: tsec: Fix offset of MDIO registers for DM_ETH

net: tsec: Fix offset of MDIO registers for DM_ETH

By convention, the eTSEC MDIO controller nodes are defined in DT at
0x2d24000 and 0x2d50000, but actually U-Boot does not touch the
interrupt portion of the register map (MDIO_IEVENTM, MDIO_IMASKM,
MDIO_EMAPM).

That leaves only the MDIO bus registers (MDIO_MIIMCFG, MDIO_MIIMCOM,
MDIO_MIIMADD, MDIO_MIIMADD, MDIO_MIIMCON, MDIO_MIIMSTAT) which start at
the 0x520 offset.

So shift the DT-defined register map by the offset of MDIO_MIIMCFG when
mapping the MDIO bus registers.

Signed-off-by: Vladimir Oltean <olteanv@gmail.com>
Acked-by: Joe Hershberger <joe.hershberger@ni.com>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>