realtek: rtl930x: add support for Sirivision SR-ST3408F

realtek: rtl930x: add support for Sirivision SR-ST3408F

This commit adds support for the Sirivision SR-ST3408F,
an 8-port 10G switch with 4x RJ45 ports and 4x SFP ports.

SoC:      Realtek RTL9303
RAM:      512 MiB DDR3
Flash:    SPI-NOR 16 MiB (Winbond)
Ethernet: 4x RJ45 via RTL8261BE PHY (10G/5G/2.5G/1G/100M/10M)
          4x SFP+
LEDs:     1x power, 1x system green
          2x RJ45 LEDs/port (link/act, 10G)
          1x SFP+ LED/port (link/act)
Button:   Reset
Console:  Cisco-style RJ45, 115200 8n1
Fan:      4040 12V fan (hardwired)
Power:    12V 3A internal PSU

OpenWrt installation
---------------------

1. Attach to serial console using a Cisco cable, 115200 8n1
2. Power on the switch and rapidly press Esc to interrupt U-Boot
3. Initialize the RJ45 ports by running `rtk network on`
4. Serve the initramfs-kernel.bin image on TFTP, IP 192.168.1.111
5. Use any RJ45 port to connect to your TFTP server
6. Run `tftpboot 0x81000000 initramfs-kernel.bin; bootm`
7. Use `mtd dump` or `dd` to backup the original firmware at `mtd5` first
8. Run sysupgrade to flash sysupgrade.bin into the switch and reboot

Restoring original firmware
-----------------------------

Vendor does not provide firmware images, so you should have made a backup.
Use `mtd write` to restore your backed up copy into the `mtd5` partition.

MAC Address
------------
All interfaces are assigned the same MAC address, from the U-Boot env
`ethaddr`. This MAC address is also printed on the device label.

Signed-off-by: Darell Tan <darell.tan@gmail.com>
Link: https://github.com/openwrt/openwrt/pull/23686
Signed-off-by: Jonas Jelonek <jelonek.jonas@gmail.com>