net: mv643xx_eth: support MII/GMII/RGMII modes for Kirkwood

Support mode switch properly, which is not available before.

If SoC has two Ethernet controllers, by setting both of them into MII
mode, the first controller enters GMII mode, while the second
controller is effectively disabled. This requires configuring (and
maybe enabling) the second controller in the device tree, even though
it cannot be used.

Signed-off-by: David Yang <mmyangfl@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>