econet: en7528: split DASAN H660GM-A into Airtel and Generic variants

econet: en7528: split DASAN H660GM-A into Airtel and Generic variants

The DASAN H660GM-A GPON ONT exists in two hardware variants, one
distributed by ISPs and one sold directly by the manufacturer [1].

Split the single DTS into a shared dtsi and per-variant DTS files
with the correct GPIO mappings, flash layout, and LED definitions.

Variant comparison:

  Feature        | Airtel                    | Generic
  ---------------+---------------------------+-------------------------
  Supplied by    | Airtel Xstream Fiber      | Retail
  Flash          | Micron MT29F2G01ABAGD     | Micron MT29F1G01ABAFD
                 | 256 MB                    | 128 MB
  Known P/N      | DPW-G-P-66GMAZ-A3         | DPW:G-P-660GMAY-G0
                 | (some units unlabelled)   | DPW:G-P-660GMAY-G3
  WPS LED        | Yes                       | No
  WLAN button    | No                        | Yes
  USB LED        | No                        | Yes
  LAN LEDs       | Green only                | Green + Amber

[1] https://dasanns.com/en/solution/detail.php?mode=view&cat_no=32&offset=&idx=122

Tested-by: Arif Rahman Hakim <arif2785id@gmail.com>
Signed-off-by: Ahmed Naseef <naseefkm@gmail.com>
Link: https://github.com/openwrt/openwrt/pull/22095
Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>