realtek: Add ZyXEL GS1900-8

realtek: Add ZyXEL GS1900-8

The ZyXEL GS1900-8 is a 8 port switch without any PoE functionality or
SFP ports, but otherwise similar to the other GS1900 switches.

Specifications
--------------
* Device:    ZyXEL GS1900-8 v1.2
* SoC:       Realtek RTL8380M 500 MHz MIPS 4KEc
* Flash:     Macronix MX25L12835F 16 MiB
* RAM:       Nanya NT5TU128M8GE-AC 128 MiB DDR2 SDRAM
* Ethernet:  8x 10/100/1000 Mbit
* LEDs:      1 PWR LED (green, not configurable)
             1 SYS LED (green, configurable)
             8 ethernet port status LEDs (green, SoC controlled)
* Buttons:   1 on-off glide switch at the back (not configurable)
             1 reset button at the right side, behind the air-vent
               (not configurable)
             1 reset button on front panel (configurable)
* Power      12V 1A barrel connector
* UART:      1 serial header (JP2) with populated standard pin connector on
             the left side of the PCB, towards the back. Pins are labelled:
             + VCC (3.3V)
             + TX (really RX)
             + RX (really TX)
             + GND
             the labelling is done from the usb2serial connector's point of
             view, so RX/ TX are mixed up.

Serial connection parameters for both devices: 115200 8N1.

Installation
------------
Instructions are identical to those for the GS1900-10HP and GS1900-8HP.

* Configure your client with a static 192.168.1.x IP (e.g. 192.168.1.10).
* Set up a TFTP server on your client and make it serve the initramfs
  image.
* Connect serial, power up the switch, interrupt U-boot by hitting the
  space bar, and enable the network:
  > rtk network on
* Since the GS1900-10HP is a dual-partition device, you want to keep the
  OEM firmware on the backup partition for the time being. OpenWrt can
  only boot off the first partition anyway (hardcoded in the DTS). To
  make sure we are manipulating the first partition, issue the following
  commands:
  > setsys bootpartition 0
  > savesys
* Download the image onto the device and boot from it:
  > tftpboot 0x84f00000 192.168.1.10:openwrt-realtek-generic-zyxel_gs1900-8-initramfs-kernel.bin
  > bootm
* Once OpenWrt has booted, scp the sysupgrade image to /tmp and flash it:
  > sysupgrade /tmp/openwrt-realtek-generic-zyxel_gs1900-8-squashfs-sysupgrade.bin

Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>