econet: add EN751627 subtarget and Zyxel EX3301-T0 board

econet: add EN751627 subtarget and Zyxel EX3301-T0 board

The EN751627 EcoNet subtarget consists of the EN7516 DSL SoC and the
(rare) EN7527 xPON SoC.

We currently support pci / wifi, usb and flash, but the EN751221 eth
driver is not portable to this family right now.

Zyxel EX3301-T0 is a wifi router based on the EN7516, it is a DSL SoC
but lacks the DSL port.

Installation instructions:
1. Serial access is required, stop the Zyxel bootloader.
2. Use ATENv3 https://github.com/cjdelisle/ATENv3 to unlock bootloader
3. "ATLD x" on the prompt to start a TFTP server
4. Connect ethernet cable from any lan (yellow) port on modem to a
device.
5. On your device, configure network to 192.168.1.2/30
6. On your device, send TRX file to 192.168.1.1 with name x, i.e.
tftp -p -l ./econet/tclinux -r x 192.168.1.1
7. On modem, you should see a line like this:
"Total 8022324 (0x7A6934) bytes received" note the hex value
8. "ATGU" to enter econet bootloader
9. "flash 80000 80020000 <the hex number without 0x>"
For example: flash 80000 80020000 7A6934
10. "reboot 1" -- start the system

If it boots back into the factory OS, you need to switch OS, from the
ZHAL prompt:

1. "ATCB" -- load data from flash
2. "ATCF 0" -- switch to OS 0
3. "ATBT 1" -- enable flash write
4. "ATSB" -- save data
5. "ATSR 1" -- reboot system

Signed-off-by: Caleb James DeLisle <cjd@cjdns.fr>
Link: https://github.com/openwrt/openwrt/pull/22945
Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>