ath79: Add support for OpenMesh MR1750 v2

ath79: Add support for OpenMesh MR1750 v2

Device specifications:
======================

* Qualcomm/Atheros QCA9558 ver 1 rev 0
* 720/600/240 MHz (CPU/DDR/AHB)
* 128 MB of RAM
* 16 MB of SPI NOR flash
  - 2x 7 MB available; but one of the 7 MB regions is the recovery image
* 3T3R 2.4 GHz Wi-Fi (11n)
* 3T3R 5 GHz Wi-Fi (11ac)
* 6x GPIO-LEDs (2x wifi, 2x status, 1x lan, 1x power)
* 1x GPIO-button (reset)
* external h/w watchdog (enabled by default))
* TTL pins are on board (arrow points to VCC, then follows: GND, TX, RX)
* 1x ethernet
  - AR8035 ethernet PHY (RGMII)
  - 10/100/1000 Mbps Ethernet
  - 802.3af POE
  - used as LAN interface
* 12-24V 1A DC
* internal antennas

Flashing instructions:
======================

Various methods can be used to install the actual image on the flash.
Two easy ones are:

ap51-flash
----------

The tool ap51-flash (https://github.com/ap51-flash/ap51-flash) should be
used to transfer the image to the u-boot when the device boots up.

initramfs from TFTP
-------------------

The serial console must be used to access the u-boot shell during bootup.
It can then be used to first boot up the initramfs image from a TFTP server
(here with the IP 192.168.1.21):

   setenv serverip 192.168.1.21
   setenv ipaddr 192.168.1.1
   tftpboot 0c00000 <filename-of-initramfs-kernel>.bin && bootm $fileaddr

The actual sysupgrade image can then be transferred (on the LAN port) to the
device via

  scp <filename-of-squashfs-sysupgrade>.bin root@192.168.1.1:/tmp/

On the device, the sysupgrade must then be started using

  sysupgrade -n /tmp/<filename-of-squashfs-sysupgrade>.bin

Signed-off-by: Sven Eckelmann <sven@narfation.org>
[rebase, add LED migration]
Signed-off-by: Adrian Schmutzler <freifunk@adrianschmutzler.de>