ramips: add support for Zyxel LTE7490-M904

The Zyxel LTE7490-M904 is an 802.3at PoE powered LTE outdoor (IP68) CPE
with integrated directional antennas.

Specifications:

- SoC: MediaTek MT7621AT
- RAM: 256 MB
- Flash: 128 MB MB NAND (MX30LF1G18AC)
- WiFi: MediaTek MT7603E 802.11b/g/n
- Switch: 1 LAN port (1 Gbps)
- LTE/3G/2G: Quectel EG18-EA LTE-A Cat. 18 connected by USB3 to SoC
- SIM: 1 micro-SIM slots under transparent cover
- Buttons: Reset, WLAN under same cover
- LEDs: Multicolour green/red/amber under same cover (visible)
- Power: 802.3at PoE via LAN port

The device is built as an outdoor ethernet to LTE bridge or router.
The wifi interface is intended for installation and/or temporary
management purposes only.

UART Serial:

57600N1, located on populated 5 pin header J5:

 [o] GND
 [ ] key - no pin
 [o] RX
 [o] TX
 [o] 3.3V Vcc

Remove the SIM/button/LED cover and 12 screws holding the back plate
and antenna cover together. Be careful with the cables.

Installation from OEM web GUI:

- Log in as "admin" on OEM web GUI
- Upload OpenWrt initramfs-recovery.bin image on the
  Maintenance -> Firmware page
- Wait for OpenWrt to boot and ssh to root@192.168.1.1
- Sysupgrade to the OpenWrt sysupgrade image and reboot

For more details about flashing see:
2449a63208 (ramips: mt7621: Add support for ZyXEL NR7101, 2021-04-19)

Main porting work done by Ernesto Castellotti <ernesto@castellotti.net>:
bf1c12f68b (ramips: add support for ZyXEL LTE7490-M904, 2023-12-20)

Signed-off-by: Eric Schäfer <eric@es86.de>
Link: https://github.com/openwrt/openwrt/pull/17485
Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>

package/boot/uboot-envtools/files/ramips

@@ -138,6 +138,7 @@ zyxel,lte3301-plus)
 	ubootenv_add_uci_config "/dev/mtd1" "0x0" "0x1000" "0x80000"
 	;;
 zyxel,lte5398-m904|\
+zyxel,lte7490-m904|\
 zyxel,nr7101)
 	idx="$(find_mtd_index Config)"
 	[ -n "$idx" ] && \