net: phy: Turn default auto-negotiation timeout into Kconfig symbol

Let users configure default auto-negotiation timeout via Kconfig
instead of specifying it in board configuration headers. This is
the first step toward converting this to Kconfig fully, so far the
legacy PHY_ANEG_TIMEOUT in configuration headers takes precedence.

Signed-off-by: Marek Vasut <marek.vasut+renesas@mailbox.org>
diff --git a/drivers/net/phy/Kconfig b/drivers/net/phy/Kconfig
index 3d96938..73064b2 100644
--- a/drivers/net/phy/Kconfig
+++ b/drivers/net/phy/Kconfig
@@ -23,6 +23,12 @@
 	help
 	  Select this if you want to control which phy address is used
 
+config PHY_ANEG_TIMEOUT
+	int "PHY auto-negotiation timeout"
+	default 4000
+	help
+	  Default PHY auto-negotiation timeout.
+
 if PHY_ADDR_ENABLE
 config PHY_ADDR
 	int "PHY address"