net: dsa: qca8k: Improve SGMII interface handling

This patch improves the handling of the SGMII interface on the QCA8K
devices. Previously the driver did no configuration of the port, even if
it was selected. We now configure it up in the appropriate
PHY/MAC/Base-X mode depending on what phylink tells us we are connected
to and ensure it is enabled.

Tested with a device where the CPU connection is RGMII (i.e. the common
current use case) + one where the CPU connection is SGMII. I don't have
any devices where the SGMII interface is brought out to something other
than the CPU.

Signed-off-by: Jonathan McDowell <noodles@earth.li>
Signed-off-by: David S. Miller <davem@davemloft.net>