ath79: add support for GL.iNet GL-MiFi

ath79: add support for GL.iNet GL-MiFi

Add support for the ar71xx supported GL.iNet GL-MiFi to ath79.

Specifications:
 - Atheros AR9331
 - 64 MB of RAM
 - 16 MB of FLASH (SPI NOR)
 - 2x 10/100/1000 Mbps Ethernet
 - 2.4GHz (AR9330), 802.11b/g/n
 - 1x USB 2.0 (vbus driven by GPIO)
 - 4x LED, driven by GPIO
 - 1x button (reset)
 - 1x mini pci-e slot (vcc driven by GPIO)

Flash instructions:

Vendor software is based on openwrt so you can flash the sysupgrade
image via the vendor GUI or using command line sysupgrade utility.
Make sure to not save configuration over reflash as uci settings
differ between versions.

Note on MAC addresses:

Even though the platform is capable to providing separate MAC addresses
to the interfaces vendor firmware does not seem to take advantage of
that. It appears that there is only single unique pre-programmed
address in the art partition and vendor firmware uses that for
every interface (eth0/eth1/wlan0). Similar behaviour has also been
implemented in this patch.

Note on GPIOs:

In vendor firmware the gpio controlling mini pci-e slot is named
3gcontrol while it actually controls power supply to the entire mini
pci-e slot. Therefore a more descriptive name (minipcie) was chosen.
Also during development of this patch it became apparent that the
polarity of the signal is actually active low rather than active high
that can be found in vendor firmware.

Acknowledgements:

This patch is based on earlier work[1] done by Kyson Lok. Since the
initial mailing-list submission the patch has been modified to comply
with current openwrt naming schemes and dts conventions.

[1] http://lists.openwrt.org/pipermail/openwrt-devel/2018-September/019576.html

Signed-off-by: Antti Seppälä <a.seppala@gmail.com>