realtek: Add support for ZyXEL GS1900-48 Switch

realtek: Add support for ZyXEL GS1900-48 Switch

The GS1900-48 is a 48 + 2 port Gigabit L2 switch with 48 gigabit ports.
Hardware:
    RTL8393M SoC
    Macronix MX25l12805D (16MB flash)
    128MB RAM
    6 * RTL8218B external PHY
    2 * RTL8231 GPIO extenders to control the port LEDs, system LED and
    Reset button

2 Uplink ports are SFP cages which support 1000 Base-X mini GBIC modules.

Power is supplied via a 230 volt mains connector.
The board has a hard reset switch SW1, which is is not reachable from the outside.
J4 provides a 12V RS232 serial connector which is connected through U8 to
the 3.3V UART of the RTL8393. Conversion is done by U8, a SIPEX 3232EC.
To connect to the UART, wires can be soldered to R603 (TX)  and R602 (RX).

Installation:
Install the squashfs image via Realtek's original Web-Interface.

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