phy: add driver for Microsemi Ocelot SerDes muxing

The Microsemi Ocelot can mux SerDes lanes (aka macros) to different
switch ports or even make it act as a PCIe interface.

This adds support for the muxing of the SerDes.

Signed-off-by: Quentin Schulz <quentin.schulz@bootlin.com>
Signed-off-by: David S. Miller <davem@davemloft.net>