ramips: add support for ZyXEL LTE3301-Plus

ramips: add support for ZyXEL LTE3301-Plus

The ZyXEL LTE3301-PLUS is an 4G indoor CPE with 2 external LTE antennas.

Specifications:

 - SoC: MediaTek MT7621AT
 - RAM: 256 MB
 - Flash: 128 MB MB NAND (MX30LF1G18AC)
 - WiFi: MediaTek MT7615E
 - Switch: 4 LAN ports (Gigabit)
 - LTE: Quectel EG506 connected by USB3 to SoC
 - SIM: 1 micro-SIM slot
 - USB: USB3 port
 - Buttons: Reset, WPS
 - LEDs: Multicolour power, internet, LTE, signal, Wifi, USB
 - Power: 12V, 1.5A

The device is built as an indoor ethernet to LTE bridge or router with
Wifi.

UART Serial:

57600N1
Located on populated 5 pin header J5:

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

MAC assignment:
lan:  98:0d:67:ee:85:54 (base, on the device back)
wlan: 98:0d:67:ee:85:55

Installation from web GUI:

- Log in as "admin" on http://192.168.1.1/
- Upload OpenWrt initramfs-recovery.bin image on the
  Maintenance -> Firmware page
- Wait for OpenWrt to boot and ssh to root@192.168.1.1
- format ubi device: ubiformat /dev/mtd6
- attach ubi device: ubiattach -m6
- create rootfs volume: ubimkvol /dev/ubi0 -n0 -N rootfs -s 1MiB
- rootfs_data volume: ubimkvol /dev/ubi0 -n1 -N rootfs_data -s 1MiB
- run sysupgrade with sysupgrade image

For more details about flashing see
commit 2449a632084b ("ramips: mt7621: Add support for ZyXEL NR7101").

Please note that this commit is needed:
firmware-utils: add marcant changes for ZyXEL NBG6716 and LTE3301-PLUS

Signed-off-by: André Valentin <avalentin@marcant.net>