ramips: add support for ZyXEL Keenetic Start

Device specification:
- SoC: RT5350F
- CPU Frequency: 360 MHz
- Flash Chip: Winbond 25Q32 (4096 KiB)
- RAM: 32768 KiB
- 5x 10/100 Mbps Ethernet (4x LAN, 1x WAN)
- 1x external, non-detachable antenna
- UART (J1) header on PCB (57800 8n1)
- Wireless: SoC-intergated: 2.4GHz 802.11bgn
- USB: None
- 3x LED, 2x button

Flash instruction:
1. Configure PC with static IP 192.168.1.2/24 and start TFTP server.
2. Rename "openwrt-ramips-rt305x-kn_st-squashfs-sysupgrade.bin"
   to "kstart_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 TFTP server, write it to flash and reboot.

Signed-off-by: Vladimir Kot <vova28rus@gmail.com>
[fixed git commit author and whitespace issues in DTS]
Signed-off-by: Petr Štetiar <ynezz@true.cz>