ramips: add support for ZyXEL LTE5398-M904

ramips: add support for ZyXEL LTE5398-M904

ZyXEL LTE5398-M904 is a dual band 802.11ac indoor LTE/3G CPE with an FXS
port.

Specifications:

* SoC: Mediatek MT7621AT
* RAM: 256 MB
* Flash: 128MB NAND (MX30LF1G18AC)
* WiFi: MediaTek MT7603 2.4G + MediaTek MT7615 5G
* Switch: 2 GbE ports MT7530
* LTE/3G: Quectel EG18-EA LTE-A Cat. 18
* SIM: 1 micro-SIM card slot
* Buttons: Reset, WPS
* LEDs: power (G/B), internet (G), LTE (R/G/Orange), WiFi (G), voice (G)
* VoIP: 1 FXS RJ11 port
* Power: 12V, 2A

UART serial console:

57600,8N1
Unpopulated header J5:

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

Installation:

* Log in as root using ssh to 192.168.1.1
* scp OpenWrt initramfs-recovery.bin image to root@192.168.1.1:/tmp/
* Prepare bootloader config by running:
   nvram setro uboot DebugFlag 0x1
   nvram setro uboot CheckBypass 0
   nvram commit
* Run "mtd_write -w write /tmp/initramfs-recovery.bin Kernel" and reboot
* Wait for OpenWrt to boot and ssh to root@192.168.1.1
* Run sysupgrade with OpenWrt squashfs-sysupgrade.bin image

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

Unsupported:

* FXS/Voice

Signed-off-by: Milan Krstic <milan.krstic@gmail.com>