Add support for K0501W boards

This board appears for example in the Davuaz Da-K6501W switch, and is
designed similarly to Hi-K0402WS.

However, it only has one SFP port and another 2.5G copper port using
RTL8221B 2.5G PHY instead. Also, some components on the board, like
additional LEDs, mode switch, and second flash chip are not populated.

Like earlier revisions of the K0402W(S) board, there is no UART, so
debugging is limited.

As this is an unmanaged switch, installation needs a flash programmer.

doc/devices/K0501W.md

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+# K0501W
+
+This board appears for example in the Davuaz Da-K6501W switch.
+
+It is designed similarly to Hi-K0402WS. However, there are a few differences:
+- One SFP port is replaced by a RTL8221B 2.5G PHY
+- Only one LED is populated for the SFP port
+- No mode switch and only one flash chip
+- Like earlier revisions of the K0402W(S) board, there is no UART
+
+Even though board looks very similar to K0402W(S), the actual GPIO and LED configuration is quite different.
+All ports and LEDs are supported.
+
+Installation is possible using a flash programmer.
+The BoyaMicro 25Q16BSSIG flash chip is supported by flashprog with chip name "B.25D16AS/BY25Q16BS/BY25Q16ES".
+
+## PCB pictures
+
+The board is marked `PCB-K0501W-V2.0 DIP-K0501WS-V2.0`.
+
+<img src="photos/K0501W/pcb-top.jpg" width="300" />
+
+<img src="photos/K0501W/pcb-bottom.jpg" width="300" />