ramips: add support for ZyXEL Keenetic 4G Rev.B Device

ramips: add support for ZyXEL Keenetic 4G Rev.B Device

Specification:
SoC: RT5350
CPU Frequency: 360 MHz
Flash Chip: Macronix MX25L6406E (8192 KiB)
RAM: Winbond W9825G6JH-6 (32768 KiB)
3x 10/100 Mbps Ethernet (2x LAN, 1x WAN)
1x external antenna
UART (J1) header on PCB (57800 8n1)
Wireless: SoC-intergated: 2.4GHz 802.11bgn
USB: Yes
8x LED, 2x button

Flash instruction:
Configure PC with static IP 192.168.99.8/24 and start TFTP server.
Rename "openwrt-ramips-rt305x-zyxel_keenetic-4g-b-squashfs-sysupgrade.bin"
to "rt305x_firmware.bin" and place it in TFTP server directory.
Connect PC with one of LAN ports, press the reset button, power up
the router and keep button pressed until power LED start blinking.
Router will download file from TFTP server, write it to flash and reboot.

Signed-off-by: Sergei Burakov <senior.anonymous@mail.ru>