ramips: add new device ZyXEL Keenetic as kn

This device exactly same as NBG-419N but with USB port and USB Led.

Specification:

- SoC: Ralink RT3052 (MIPS24Kc) @384MHz
- RAM: 32 MiB
- Flash: 8 MiB
- WLAN: WiSoC 2T2R/300Mbps (2.4GHz)
- LAN: 4x100M
- WAN: 1x100M
- USB: 1x2.0

Installation via serial console (57600 8N1) from TFTP server
 - rename the firmware to something shorter, for example
   "sysupgrade.bin" (max. 32 chars)
 - copy firmware TFTP server's directory
 - when you power on device, and see U-Boot log, immediatly push "2"
   once.
 - You will see this message:
     2: System Load Linux Kernel then write to Flash via TFTP.
        Warning!! Erase Linux in Flash then burn new one. Are you sure?
 - Push "y", and enter: device IP, then TFTP server's IP, and then
   image firmware file name.

The firmware will be downloaded within ~30 seconds and flashed to the
device (It will take about 2 minutes).

Signed-off-by: Alexey Belyaev <spider@spider.vc>
[squash commits, compact commit message, fix compatible string, remove
superfluous pinmuxes]
Signed-off-by: Mathias Kresin <dev@kresin.me>