phy: armada38x: add common phy support

Add support for the Armada 38x common phy to allow us to change the
speed of the Ethernet serdes lane.  This driver only supports
manipulation of the speed, it does not support configuration of the
common phy.

Signed-off-by: Russell King <rmk+kernel@armlinux.org.uk>
Signed-off-by: David S. Miller <davem@davemloft.net>