ath79: add support for Dell SonicPoint ACe APL26-0AE

ath79: add support for Dell SonicPoint ACe APL26-0AE

Dell/SonicWall APL26-0AE (marketed as SonicPoint ACe) is a dual band
wireless access point. End of life as of 2022-07-31.

Specification
SoC: QualcommAtheros QCA9550
RAM: 256 MB DDR2
Flash: 32 MB SPI NOR
WIFI: 2.4 GHz 3T3R integrated
      5 GHz 3T3R QCA9890 oversized Mini PCIe card
Ethernet: 2x 10/100/1000 Mbps QCA8334
          port labeled lan1 is PoE capable (802.3at)
USB: 1x 2.0
LEDs: LEDs: 6x which 5 are GPIO controlled and two of them are dual color
Buttons: 2x GPIO controlled
Serial: RJ-45 port, SonicWall pinout
        baud: 115200, parity: none, flow control: none

Before flashing, be sure to have a copy of factory firmware, in case You
wish to revert to original firmware.
All described procedures were done in following environment:
ROM Version: SonicROM (U-Boot) 8.0.0.0-11o
SafeMode Firmware Version: SonicOS 8.0.0.0-14o
Firmware Version: SonicOS 9.0.1.0
In case of other versions, following installation instructions might be
ineffective.

Installation
1. Prepare TFTP server with OpenWrt sysupgrade image and rename that
   image to "sp_fw.bin".
2. Connect to one of LAN ports.
3. Connect to serial port.
4. Hold the reset button (small through hole on side of the unit),
   power on the device and when prompted to stop autoboot, hit any key.
   The held button can now be released.
5. Alter U-Boot environment with following commands:
    setenv bootcmd bootm 0x9F110000
    saveenv
6. Adjust "ipaddr" (access point, default is 192.168.1.1) and "serverip"
   (TFTP server, default is 192.168.1.10) addresses in U-Boot
   environment, then run following commands:
    tftp 0x80060000 sp_fw.bin
    erase 0x9F110000 +0x1EF0000
    cp.b 0x80060000 0x9F110000 $filesize
7. After successful flashing, execute:
    boot
8. The access point will boot to OpenWrt. Wait few minutes, until the
    wrench LED will stop blinking, then it's ready for configuration.

Known issues
Initramfs image can't be bigger than specified kernel size, otherwise
bootloader will throw LZMA decompressing error. Switching to lzma-loader
should workaround that.
This device has Winbond 25Q256FVFG and doesn't have reliable reset, which
causes hang on reboot, thus broken-flash-reset needs to be added. This
property addition causes dispaly of "scary" warning on each boot, take
this warnig into consideration.

Signed-off-by: Tomasz Maciej Nowak <tmn505@gmail.com>