realtek: add support for Linksys LGS310C

realtek: add support for Linksys LGS310C

Hardware specification
----------------------

* RTL8380M SoC, 1 MIPS 4KEc core @ 500MHz
* 256MB DRAM
* 32MB NOR Flash
* 8 x 10/100/1000BASE-T ports
* 2 x SFP ports
* Power LED, Fault LED
* Reset button on front panel
* UART (115200 8N1) via populated standard pin header marked JP1

TODO: The SFP ports use a shared SCL GPIO that the driver cannot handle.
The left SFP port (lan9) is defined and fully functional while the laser
on the right SFP port (lan10) is off by default.

UART pinout
-----------

[o]ooo|JP1
 | ||`------ GND
 | |`------- RX
 | `-------- TX
 `---------- Vcc (3V3)

Installation using OEM webinterface
-----------------------------------

1. Make sure you are running OEM firmware in secondary slot
2. Install squashfs-factory.imag to primary slot by upload via http

Installation using serial interface
-----------------------------------

1. Press "a" "c" "p" during message "Enter correct key to stop autoboot"
2. Load image with "upgrade runtime <TFTP IP>:squashfs-sysupgrade.bin" command
3. Switch to primary slot with "setsys bootpartition 0"
4. Store config with "savesys"
5. Boot the image with `boota` command

Dual-boot with stock firmware using writable u-boot-env
-------------------------------------------------------

From stock to OpenWrt / primary image 1 (CLI as admin):
   - > boot system image1
   - > reboot

From OpenWrt to stock / boot image 2: (shell as root)
   - # fw_setsys bootpartition 1
   - # reboot

Debrick using serial interface
------------------------------

1. Press "a" "c" "p" during message "Enter correct key to stop autoboot"
2. Load vendor image with "upgrade runtime <TFTP IP>:LGS310xxxxx.imag"
3. switch to primary partition "setsys bootpartition 0"
4. safe config "savesys"

Further documentation
---------------------
See https://openwrt.org/toh/linksys/lgs352c

It has been developed and tested on device with v1 revision.

Signed-off-by: Markus Stockhausen <markus.stockhausen@gmx.de>
Link: https://github.com/openwrt/openwrt/pull/16068
[Add missing 'w' in name of firmware partition]
Signed-off-by: Sander Vanheule <sander@svanheule.net>