realtek: add support for XikeStor SKS7300-4X4T

realtek: add support for XikeStor SKS7300-4X4T

The XikeStor SKS7300-4X4T is a 10GbE switch with 4x copper ports and
4x SFP slots. It has a built-in console port and is powered by a RTL9303
SoC.

Specifications:
---------------

  * Soc: RTL9303
  * Flash: 32 MiB SPI flash
  * RAM: 512 MiB
  * Ethernet: 8x 10/100/1000/2500/5000/10000 Mbps
  * Buttons: 1x Reset
  * UART: CISCO console ports on the front, 115200, 8n1
  * controllable fan, integrated temperature sensor

Not yet enabled:
----------------
  * Fan control: it's controlled via simple i2c registers, but no driver
                 has been written yet.

Installation:
-------------

This device uses an obfuscated bootloader and an obfuscated image. As such,
the installation can only be performed using the console ports.

1. Set the switch to boot from the first image.

2. Attach console cable and hold Ctrl+C while powering on the switch

3. After a few seconds, a very basic U-Boot menu appears. Wait for the user
   input to appear, then press "z" to get to the message "Please input auth
   code".

4. Type "jiangks" as the password, the RTL9300 prompt appears.

5. Load the OpenWrt initramfs image via TFTP:

   > setenv serverip 192.168.0.1
   > setenv ipaddr 192.168.0.2
   > tftpboot 0x83000000 openwrt.bin
   > bootm 0x83000000

6. Once OpenWrt has booted, use the "sysupgrade" image to perform the
   actual installation.

7. Reboot the switch and enjoy OpenWrt.

Recovery/return to stock:
-------------------------

Flash an OEM firmware upgrade file via sysupgrade.

Signed-off-by: Andreas Böhler <dev@aboehler.at>
Link: https://github.com/openwrt/openwrt/pull/23305
Signed-off-by: Robert Marko <robimarko@gmail.com>