net: mvmdio: add xmdio xsmi support

This patch adds the xmdio xsmi interface support in the mvmdio driver.
This interface is used in Ethernet controllers on Marvell 370, 7k and 8k
(as of now). The xsmi interface supported by this driver complies with
the IEEE 802.3 clause 45. The xSMI interface is used by 10GbE devices.

Signed-off-by: Antoine Tenart <antoine.tenart@free-electrons.com>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>