net: phy: at803x: add fiber support

Previously this driver always forced the copper page to be selected,
however for AR8031 in 100Base-FX or 1000Base-X modes, the fiber page
needs to be selected. Set the appropriate mode based on the hardware
mode_cfg strap selection.

Enable the appropriate interrupt bits to detect fiber-side link up
or down events.

Update config_aneg and read_status methods to use the appropriate
Clause 37 calls when fiber mode is in use.

Signed-off-by: Robert Hancock <robert.hancock@calian.com>
Signed-off-by: David S. Miller <davem@davemloft.net>