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

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>
(cherry picked from commit b90ea19860853dd538e704e3e4402686c316e43c)