Add support for the SWTG018AS-A V2.0 8+1 unmanged switch

The SWTG018AS-A V2.0 is an 8x2.5GBit/s port + 1 SFP+ port unmanged device
It is sold in an unbranded casing e.g. by Ampcom.
Hardware:
SoC: RTL8373N
Additional PHY: RTL8224
Flash: Puya P25D40SH (4MBit, slimmer, not standard SOP-8 package!)
LEDs: 1 Green + 1 Orange per Ethernet port (green designates 2.5GBit)
      1 Green LED for the SFP+ port

The photos already show a modified PCB: Unpopulated resistors R52 and R53
have been bridged in order to enable the serial console. The smaller flash
memory chip at U2 has been replaced with a Windbond 128MBit standard-sized
SOIC-8 chip in order to easily use SOIC clamps to flash the device. Also
the serial header has been populated.

doc/devices/SWTG018AS_A_V2_0.md

@@ -0,0 +1,54 @@
+# SWTG018AS-A V2.0
+
+The following is a documentation for the unmanaged switch marked as `SWTG018AS-A V2.0`.
+It is e.g. sold under the Ampcom brand, but no branh-markings are found on the device.
+
+The original software is running UART on 9600 baud rate. 
+
+Using a SOIC clamp in-board is the only method for initial installation.
+
+The board has a single flash chips `BS` with 4M-bit size. The front switch, switches between the two flash chips.
+These can be programed independently by using said switch - so it is e.g. possible to run the original and new firmware in parallel.
+
+### Label specifications
+
+- **Name**: 9-Ports 2.5G Ethernet Switch
+- **Ports**:
+  - 8 × RJ45: 10/100/1000/2500 Mbps
+  - 1 × SFP+: 1000 / 2500 / 10000 Mbps
+- **Power**: 12V DC, 1A barrel connector 
+
+<img src="photos/SWTG018AS_A_V_2_0/label.jpg" width="300" />
+
+### What works
+The device is fully supported:
+- All 8 2.5GBASE-T RJ45 ports work at 10/100/1000/2500 Mbps
+- The SFP+ port supports 1G, 2.5G and 10G modules 
+- LEDs work with the same indiciations as the OEM firmware
+
+### PCB overview
+
+**Board markings**
+- Top silkscreen: SWTG018AS-A-V2.0.1_19649
+
+Top side
+
+<img src="photos/SWTG018AS_A_V_2_0/pcb_top.jpg" width="300" />
+
+Bottom
+
+<img src="photos/SWTG018AS_A_V_2_0/pcb_bottom.jpg" width="300" />
+
+### J1, serial console
+
+| `J1` pin | Signal      |
+| -------- | ----------- |
+| 1        | 3V3         |
+| 2        | GND         |
+| 3        | RX (Input) |
+| 4        | TX (Output)  |
+
+
+## Power supply
+
+Input power is delivered via barell plug, `12V 1A` adapter was provided.