powerpc/83xx: Fix missing #{address,size}-cells in mpc8313erdb.dts

commit b31a1d8b

 ("gianfar: Convert
gianfar to an of_platform_driver") introduced a child node for
the ethernet@25000 controller, but no address and size cells
specifiers were added, and that makes dtc unhappy:

DTC: dts->dtb  on file "arch/powerpc/boot/dts/mpc8313erdb.dts"
Warning (reg_format): "reg" property in /soc8313@e0000000/ethernet@25000/mdio@25520 has invalid length (8 bytes) (#address-cells == 2, #size-cells == 1)
Warning (avoid_default_addr_size): Relying on default #address-cells value for /soc8313@e0000000/ethernet@25000/mdio@25520
Warning (avoid_default_addr_size): Relying on default #size-cells value for /soc8313@e0000000/ethernet@25000/mdio@25520

Signed-off-by: Anton Vorontsov <avorontsov@ru.mvista.com>
Signed-off-by: Kumar Gala <galak@kernel.crashing.org>