ramips: add support for ZyXEL Keenetic Extra II

ramips: add support for ZyXEL Keenetic Extra II

Specification:
- System-On-Chip: MT7628N/N
- CPU/Speed: 580 MHz
- Flash-Chip: Winbond w25q256
- Flash size: 32768 KiB
- RAM: 128 MiB
- 5x 10/100 Mbps Ethernet
- 4x external, non-detachable antennas
- UART (J1) header on PCB (57600 8n1)
- Wireless No1 (2T2R): SoC-integrated: MT7628N 2.4GHz 802.11bgn
- Wireless No2 (2T2R): On-board chip: MT7612EN 5GHz 802.11ac
- USB: Yes 1 x 2.0
- 4x LED, 3x button

The device supports dual boot mode. So we use only first half of flash.

Flash instruction:

The only way to flash OpenWrt image is to use
tftp recovery mode in U-Boot:

1. Configure PC with static IP 192.168.1.2/24 and tftp server.
2. Rename "openwrt-ramips-mt76x8-zyxel_keenetic-extra-ii-squashfs-factory.bin"
   to "kextra2_recovery.bin" and place it in tftp server directory.
3. Connect PC with one of LAN ports, press the reset button, power up
   the router and keep button pressed until power led start blinking.
4. Router will download file from server, write it to flash and reboot.

Signed-off-by: Maxim Anisimov <maxim.anisimov.ua@gmail.com>