add deprecation notice for bktr driver.

We plan to remove bktr(4) in FreeBSD 13. Document that here.

Relnotes: yes

share/man/man4/bktr.4

@@ -1,7 +1,7 @@
 .\"
 .\" $FreeBSD$
 .\"
-.Dd January 18, 2006
+.Dd February 25, 2020
 .Dt BKTR 4
 .Os
 .Sh NAME
@@ -13,6 +13,12 @@
 .Cd device iicbus
 .Cd device iicbb
 .Cd device smbus
+.Sh DEPRECATION NOTICE
+The
+.Nm
+driver is not present in
+.Fx 13.0
+and later.
 .Sh DESCRIPTION
 The
 .Nm bktr

sys/dev/bktr/bktr_os.c

@@ -464,6 +464,7 @@ bktr_attach( device_t dev )
 	}
 #endif
 
+	gone_in_dev(dev, 13, "bktr driver removed in FreeBSD 13.0\n");
 	return 0;
 
 fail: