realtek: Add support for ZxXEL XGS1250-12 Switch

realtek: Add support for ZxXEL XGS1250-12 Switch

The ZyXEL XGS1250-12 Switch is a 11 + 1 port multi-GBit switch with
8 x 1000BaseT, 3 x 1000/2500/5000/10000BaseT Ethernet ports and
1 SFP+ module slot.

Hardware:
 - RTL9302B SoC
 - Macronix MX25L12833F (16MB flash)
 - Nanja NT5CC64M16GP-1 (128MB DDR3 SDRAM)
 - RTL8231 GPIO extender to control the port LEDs
 - RTL8218D 8x Gigabit PHY
 - Aquantia AQR113c 1/2.5/5/10 Gigabit PHYs
 - SFP+ 10GBit slot

Power is supplied via a 12V 2A standard barrel connector. At the
right side behind the grid is UART serial connector. A Serial
header can be connected to from the outside of the switch trough
the airvents with a standard 2.54mm header.
Pins are from top to bottom Vcc(3.3V), TX, RX and GND. Serial
connection is via 115200 baud, 8N1.

A reset button is accessble through a hole in the front panel

At the time of this commit, all ethernet ports work under OpenWRT,
including the various NBaseT modes, however the 10GBit SFP+ slot is not
supported.

Installation
--------------
* Connect serial as per the layout above. Connection parameters: 115200 8N1.
* Navigate to 'Management' in the OEM web interface and click on 'Firmware upgrade'
  to the left.
* Upload the OpenWrt initramfs image, and wait till the switch reboots.
* Connect to the device through serial and change the U-boot boot command.
  > fw_setenv bootcmd 'rtk network on; boota'
* Reboot, scp the sysupgrade image to /tmp, verify the checksum and flash it:
  > sysupgrade /tmp/openwrt-realtek-rtl930x-zyxel_xgs1250-12-squashfs-sysupgrade.bin
* Upon reboot, you have a functional OpenWrt installation. Leave the bootcmd
  value as is - without 'rtk network on' the switch will fail to initialise
  the network.

Web recovery
------------
The XGS1250-12 has a handy web recovery that will load when U-boot does
not find a bootable kernel. In case you would like to trigger the web
recovery manually, partially overwrite the firmware partition with some
zeroes:

  # dd if=/dev/zero of=/dev/mtd5 bs=1M count=2

If you have serial connected you'll see U-boot will start the web recovery
and print it's listening on 192.168.1.1, but by default it seems to be on
the OEM default IP for the switch - 192.168.1.3. The web recovery only
listens on HTTP (80) and *not* on 443 (HTTPS) unlike the web UI.

Return to stock
---------------
You can flash the ZyXEL firmware images to return to stock:

  # sysupgrade -F -n XGS1250-12_Firmware_V1.00(ABWE.1)C0.bix

Signed-off-by: Birger Koblitz <git@birger-koblitz.de>