realtek: add support for Xikestor SKS8300-12E2T2X

realtek: add support for Xikestor SKS8300-12E2T2X

Add board support for the Xikestor SKS8300-12E2T2X switch.

Hardware specifications:
========================
 -Realtek RTL9302C SoC, 1x MIPS-34Kc, 800 MHz
 -512 MB DDR3 RAM
 -32 MB SPI-NOR Flash
 -12x 2.5GBASE-T Ports (RTL8224)
 -2x 10GBASE-T Ports (RTL8261)
 -2x 10G SPF+ Ports
 -Reset Button on the front panel
 -Power & SYS LED's
 -UART (115200 8N1) via RJ45

Flash instruction:
==================
 -Prepare TFTP server & connect to serial port
 -Connect your computer to one of the RJ45 ports
 -Power on and interrupt autoboot with Shift + A.
 -Use Shift + Q to drop from vendor CLI to U-Boot CLI.
 -Change U-Boot Bootcommand (needed for network functionality):
	> setenv bootcmd 'rtk network on; boota'
	> saveenv
 -Start network:
	> rtk network on
 -Set switch IP and TFTP server IP:
	> setenv ipaddr [IP-ADDRESS]
	> setenv serverip [IP-ADDRESS]
 -Load initramfs image from TFTP server:
	> tftpboot 0x82000000 [IMAGEFILE]
 -Boot with the downloaded image:
	> bootm 0x82000000
 -Backup the stock firmware if needed
 -Perform sysupgrade with the sysupgrade image (in Luci or Terminal via scp & sysupgrade)

Back to stock firmware:
=======================
 -In the Terminal enter:
	> fw_setenv bootcmd 'boota'
 -Write firmware with:
	> sysupgrade -F [IMAGEFILE]

Signed-off-by: Michael Wagenhofer <michael@wagenhofer.de>
Link: https://github.com/openwrt/openwrt/pull/21773
Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>