econet: en7528: add support for DASAN H660GM-A

econet: en7528: add support for DASAN H660GM-A

DASAN H660GM-A is a GPON ONT.

Specification:
- SoC:      EcoNet EN7528
- RAM:      256 MB
- Flash:    256 MB SPI NAND
- Ethernet: 4x 1GbE
- FXS:      1 port
- GPON:     1 port
- USB:      1x USB 2.0

Install via OEM web UI:
1. Login to the factory web UI with username "superuser"
   and password "Dz$!A!r7".
2. Rename the OpenWrt image
   openwrt-econet-en7528-dasan_h660gm-a-squashfs-tclinux.trx
   to G_ONU_openwrt.bin.
3. Upload the renamed image via the firmware upgrade page
   at Maintenance > Firmware Upgrade in the factory web UI.

Signed-off-by: Ahmed Naseef <naseefkm@gmail.com>
Link: https://github.com/openwrt/openwrt/pull/21326
Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>