net: mvmdio: doc: mvmdio now used by mv643xx_eth

Amend the documentation in the mvmdio driver to note the fact
that it is now used by both the mvneta and mv643xx_eth drivers.

Signed-off-by: Leigh Brown <leigh@solinno.co.uk>
Signed-off-by: David S. Miller <davem@davemloft.net>