net: phy: dp83867: add TI PHY loopback

The existing genphy_loopback() is not working for TI DP83867 PHY as it
will disable autoneg support while another side is still enabling autoneg.
This is causing the link is not established and results in timeout error
in genphy_loopback() function.

Thus, based on TI PHY datasheet, introduce a TI PHY loopback function by
just configuring BMCR_LOOPBACK(Bit-9) in MII_BMCR register (0x0).

Tested working on TI DP83867 PHY for all speeds (10/100/1000Mbps).
Signed-off-by: Tan Tee Min <tee.min.tan@linux.intel.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Link: https://lore.kernel.org/r/20221108101527.612723-1-michael.wei.hong.sit@intel.comSigned-off-by: Jakub Kicinski <kuba@kernel.org>

drivers/net/phy/dp83867.c

@@ -933,6 +933,12 @@ static void dp83867_link_change_notify(struct phy_device *phydev)
 	}
 }
 
+static int dp83867_loopback(struct phy_device *phydev, bool enable)
+{
+	return phy_modify(phydev, MII_BMCR, BMCR_LOOPBACK,
+			  enable ? BMCR_LOOPBACK : 0);
+}
+
 static struct phy_driver dp83867_driver[] = {
 	{
 		.phy_id		= DP83867_PHY_ID,
@@ -959,6 +965,7 @@ static struct phy_driver dp83867_driver[] = {
 		.resume		= genphy_resume,
 
 		.link_change_notify = dp83867_link_change_notify,
+		.set_loopback	= dp83867_loopback,
 	},
 };
 module_phy_driver(dp83867_driver);