net: mvmdio: get and enable optional clock

Marvell mdio driver uses internal registers that can be clock gated on
some SoCs. This patch just adds optional clock handling, to allow to pass
and enable the corresponding clock.

Signed-off-by: Sebastian Hesselbarth <sebastian.hesselbarth@gmail.com>
Acked-by: Florian Fainelli <florian@openwrt.org>
Signed-off-by: David S. Miller <davem@davemloft.net>