ath79: Add support for Sophos AP15C

ath79: Add support for Sophos AP15C

The Sophos AP15C uses the same hardware as the AP15, but has a reset button.

Based on:
commit 6f1efb289837 ("ath79: add support for Sophos AP100/AP55 family")
author    Andrew Powers-Holmes <andrew@omnom.net>
          Fri, 3 Sep 2021 15:53:57 +0200 (23:53 +1000)
committer Hauke Mehrtens <hauke@hauke-m.de>
          Sat, 16 Apr 2022 16:59:29 +0200 (16:59 +0200)

Unique to AP15C:
 - Reset button
 - External RJ45 serial console port

Flashing instructions:

This firmware can be flashed either via a compatible Sophos SG or XG
firewall appliance, which does not require disassembling the device, or via
the U-Boot console available on the internal UART header.

To flash via XG appliance:
 - Register on Sophos' website for a no-cost Home Use XG firewall license
 - Download and install the XG software on a compatible PC or virtual
   machine, complete initial appliance setup, and enable SSH console access
 - Connect the target AP device to the XG appliance's LAN interface
 - Approve the AP from the XG Web UI and wait until it shows as Active
   (this can take 3-5 minutes)
 - Connect to the XG appliance over SSH and access the Advanced Console
   (Menu option 5, then menu option 3)
 - Run `sudo awetool` and select the menu option to connect to an AP via
   SSH. When prompted to enable SSH on the target AP, select Yes.
 - Wait 2-3 minutes, then select the AP from the awetool menu again. This
   will connect you to a root shell on the target AP.
 - Copy the firmware to /tmp/openwrt.bin on the target AP via SCP/TFTP/etc
 - Run `mtd -r write /tmp/openwrt.bin astaro_image`
 - When complete, the access point will reboot to OpenWRT.

To flash via U-Boot serial console:
 - Configure a TFTP server on your PC, and set IP address 192.168.99.8 with
   netmask 255.255.255.0
 - Copy the firmware .bin to the TFTP server and rename to 'uImage_AP15C'
 - Open the target AP's enclosure and locate the 4-pin 3.3V UART header [4]
 - Connect the AP ethernet to your PC's ethernet port
 - Connect a terminal to the UART at 115200 8/N/1 as usual
 - Power on the AP and press a key to cancel autoboot when prompted
 - Run the following commands at the U-Boot console:
    - `tftpboot`
    - `cp.b $fileaddr 0x9f070000 $filesize`
    - `boot`
 - The access point will boot to OpenWRT.

Signed-off-by: David Lutz <kpanic@hirnduenger.de>
(cherry picked from commit a7abc7ec3b61be553f33c49e5acc90a4d539d992)