ath79: Restore GL.iNet GL-AR300M-Lite first-boot connectivity

The relationship between GMAC0 and GMAC1 and the kernel devices
eth0 and eth1 was reversed for many ath79 devices by commit 8dde11d521
ath79: dts: drop "simple-mfd" for gmacs in SoC dtsi

The GL-AR300M-Lite is a single-port device, with the "LAN" port of the
GL-AR300M board unpopulated and its sole port now referenced as eth1,
as a result of commit 8dde11d521. The device was unreachable on
first boot or fresh config.

By changing &eth1 (GMAC1) to an MFD, GMAC0 is able to associate with
the phy and is known by the kernel as "eth0".

Thanks to Chuanhong Guo for the suggestion of "simple-mfd"

Signed-off-by: Jeff Kletsky <git-commits@allycomm.com>

target/linux/ath79/dts/qca9531_glinet_gl-ar300m-lite.dts

@@ -7,7 +7,11 @@
 	model = "GL.iNet GL-AR300M-Lite";
 };
 
-// GL-AR300M-Lite has different LED colors than the non-Lite version
+// "Disable" unpopulated GMAC1
+
+&eth1 {
+	compatible = "syscon", "simple-mfd";
+};
 
 &led_status {
 	label = "gl-ar300m-lite:red:status";