ath79-generic: Add support for OpenMesh OM2P v1

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

* Qualcomm/Atheros AR7240 rev 2
* 350/350/175 MHz (CPU/DDR/AHB)
* 32 MB of RAM
* 16 MB of SPI NOR flash
  - 2x 7 MB available; but one of the 7 MB regions is the recovery image
* 2x 10/100 Mbps Ethernet
* 1T1R 2.4 GHz Wi-Fi
* 6x GPIO-LEDs (3x wifi, 2x ethernet, 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)
* 2x fast ethernet
  - eth0
    + 18-24V passive POE (mode B)
    + used as WAN interface
  - eth1
    + builtin switch port 4
    + used as LAN interface
* 12-24V 1A DC
* external antenna

The device itself requires the mtdparts from the uboot arguments to
properly boot the flashed image and to support dual-boot (primary +
recovery image). Unfortunately, the name of the mtd device in mtdparts is
still using the legacy name "ar7240-nor0" which must be supplied using the
Linux-specfic DT parameter linux,mtd-name to overwrite the generic name
"spi0.0".

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

Gluon image name change
=======================

The device had the image name "openmesh-om2p" in older versions of Gluon.
This had to be changed with the new name in the device trees of the ath79
device tree.