net: mvpp2: fix lookup of mdio registers base address

Current mdio base lookup code relies on a 'reg' property at the upper CP
node. There is no 'reg' property there in current DT files of Armada
CP110. Use ofnode_get_addr() instead since it provides proper DT address
translation.

Cc: Ken Ma <make@marvell.com>
Cc: Stefan Chulski <stefanc@marvell.com>
Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Reviewed-by: Stefan Roese <sr@denx.de>
Acked-by: Joe Hershberger <joe.hershberger@ni.com>