Add RMII example

examples/rp2040/pico-rmii/README.md

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+
+# RMII with the PIO in an RP2040 for 10Mbps Ethernet
+
+- Full-duplex 10Mbps
+- RMII Tx implemented with a PIO state machine and single-buffer DMA transfers
+  - Once the DMA starts sending to the PIO, subsequent attempts to tx will block 
+- RMII Rx implemented with a PIO state machine and ping-pong buffer DMA transfers
+  - PIO code handles the CSR_DV signal, stops and raises an IRQ on frame ending. CRC is calculated later for minimum dead time between frames
+- SMI implemented with a PIO state machine
+- The RP2040 is clocked from the LAN8720 and works at 50MHz
+- CRC calculation uses a fast table-based algorithm that doesn't require much storage (nibble calculation)
+
+## Hardware connections
+
+You can change most pins at will, bear in mind that each function requires consecutive pins, see main.c
+The RETCLK (should be REFCLK...) pin can go to either GPIO20 or GPIO22
+
+| LAN8720 | | Raspberry Pi Pico |  |
+| ----------- | ----------- |----------------- | -- |
+| 1,2 | VCC | 3V3 | 36 |
+| 3,4 | GND | GND | 38 |
+| 5 | MDC | GPIO15 | 20 |
+| 6 | MDIO | GPIO14 | 19 |
+| 7 | CRS | GPIO8 | 11 |
+| 8 | nINT / RETCLK | GPIO20 | 26 |
+| 9 | RX1 | GPIO7 | 10 |
+| 10 | RX0 | GPIO6 | 9 |
+| 11 | TX0 | GPIO10 | 14 |
+| 12 | TX-EN | GPIO12 | 16 |
+| 13 |
+| 14| TX1 | GPIO11 | 15 |
+
+
+## Build and run
+
+Clone Mongoose repo, go to this example, and build it:
+
+```sh
+git clone https://github.com/cesanta/mongoose
+cd mongoose/examples/rp2040/pico-rmii
+make build
+```
+
+This will generate a firmware file: `build/firmware.uf2`. Reboot your Pico board in bootloader mode, and copy it to the RPI disk.
+
+Attach to the serial console, wait a couple of seconds, then plug in your Ethernet cable:
+
+```
+$ picocom /dev/ttyACM0 -i -b 115200 --imap=lfcrlf
+6213 1 mongoose.c:6757:onstatechange    Link up
+6217 3 mongoose.c:6840:tx_dhcp_discover DHCP discover sent
+621b 3 mongoose.c:6723:arp_cache_add    ARP cache: added 192.168.69.2 @ 52:54:...
+6222 2 mongoose.c:6749:onstatechange    READY, IP: 192.168.69.243
+6227 2 mongoose.c:6750:onstatechange           GW: 192.168.69.1
+622b 2 mongoose.c:6752:onstatechange           Lease: 21600 sec
+```
+
+Note the acquired IP address printed (you can also check your DHCP server). Point your browser to that address, or run curl, you should see an "ok" message:
+
+```
+$ curl 192.168.69.243
+ok
+```