realtek: add support for XikeStor SKS8310-8X

realtek: add support for XikeStor SKS8310-8X

XikeStor (Seeker) SKS8310-8X is a 8 ports Multi-Gig switch, based on
RTL9303.

Specifications:

- SoC              : Realtek RTL9303
- RAM              : DDR3 512 MiB
- Flash            : SPI-NOR 32 MiB (Macronix)
- Ethernet         : 8x 1/2.5/10 Gbps (SFP+)
- LEDs/Keys (GPIO) : 1x/1x
- UART             : "Console" port on the front panel
  - type           : RS-232C
  - connector      : RJ-45
  - settings       : 115200 8N1
- Power            : 12 VDC, 2 A

Flash instruction using initramfs image:

 1. Prepare TFTP server & connect to serial port.
 2. Connect your computer to one of the ports on SKS8310-8X with a
    suitable SFP module (some work, some don't).
 3. Power on SKS8310-8X and interrupt autoboot with Shift + A.
 4. Use Shift + Q to drop from vendor CLI to U-Boot CLI.
 5. Enable networking within U-Boot.
	> rtk network on
 6. Set switch IP and TFTP server IP (optional, adjust to your setup).
	> setenv ipaddr <ip>
	> setenv serverip <ip>
 7. Download initramfs image from TFTP server.
	> tftpboot 0x82000000 <image name>
 8. Boot with the downloaded image.
	> bootm 0x82000000
 9. With rambooted OpenWrt, backup the stock firmware if needed.
10. Copy sysupgrade image to the device.
11. Perform sysupgrade with the sysupgrade image.
12. After reboot, you should have functional OpenWrt.

Reverting to stock firmware:

 1. Download latest firmware from XikeStor and upload to your device.
 1. Write firmware with 'sysupgrade -F'.
 2. After reboot, stock firmware should boot automatically.

Co-authored-by: Jonas Jelonek <jelonek.jonas@gmail.com>
Signed-off-by: Alexandra Alth <alexandra@alth.de>
Signed-off-by: Jonas Jelonek <jelonek.jonas@gmail.com>
Link: https://github.com/openwrt/openwrt/pull/19782
Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>