net: dsa: mv88e6xxx: support in-band signalling on SGMII ports with external PHYs

If an external PHY is connected via SGMII and uses in-band signalling
then the auto-negotiated values aren't propagated to the port,
resulting in a broken link. See discussion in [0]. This patch adds
this propagation. We need to call mv88e6xxx_port_setup_mac(),
therefore export it from chip.c.

Successfully tested on a ZII DTU with 88E6390 switch and an
Aquantia AQCS109 PHY connected via SGMII to port 9.

[0] https://marc.info/?t=155130287200001&r=1&w=2

Signed-off-by: Heiner Kallweit <hkallweit1@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/dsa/mv88e6xxx/serdes.h

@@ -69,6 +69,14 @@
 #define MV88E6390_SGMII_INT_SYMBOL_ERROR	BIT(8)
 #define MV88E6390_SGMII_INT_FALSE_CARRIER	BIT(7)
 #define MV88E6390_SGMII_INT_STATUS	0xa002
+#define MV88E6390_SGMII_PHY_STATUS	0xa003
+#define MV88E6390_SGMII_PHY_STATUS_SPEED_MASK	GENMASK(15, 14)
+#define MV88E6390_SGMII_PHY_STATUS_SPEED_1000	0x8000
+#define MV88E6390_SGMII_PHY_STATUS_SPEED_100	0x4000
+#define MV88E6390_SGMII_PHY_STATUS_SPEED_10	0x0000
+#define MV88E6390_SGMII_PHY_STATUS_DUPLEX_FULL	BIT(13)
+#define MV88E6390_SGMII_PHY_STATUS_SPD_DPL_VALID BIT(11)
+#define MV88E6390_SGMII_PHY_STATUS_LINK		BIT(10)
 
 int mv88e6390x_serdes_get_lane(struct mv88e6xxx_chip *chip, int port);
 int mv88e6341_serdes_power(struct mv88e6xxx_chip *chip, int port, bool on);