Add PID 1209:2351 for PicoI2S - #1308

PicoI2S is open-source (MIT) USB audio firmware for the RP2350: a USB Audio Class 2.0 interface that drives I2S DACs and classic dual-line multibit DACs through on-chip oversampling filters, and S/PDIF, with an S/PDIF input.

Source: https://github.com/dpandov/PicoI2S (MIT, LICENSE in the repository root)

Why it needs a PID of its own, although it is firmware only:

• It runs on off-the-shelf boards (Waveshare RP2350-One, Raspberry Pi Pico 2); its users flash a release file and wire a DAC, they do not design hardware and cannot request IDs themselves.
• It is a USB audio device with a picotool reset interface (WinUSB through MS OS 2.0 descriptors), so it needs a stable ID for Windows to bind the drivers, and the host tools in the repository find the device by it.

One PID covers both modes of the firmware: the USB Audio Class 2.0 mode and a USB Audio Class 1.0 compatibility mode selected by a jumper at boot. The UAC1 mode reports a different device release number, so hosts re-read it after a mode change.

The firmware currently uses the test PIDs 1209:0001 / 1209:0002; it moves to 1209:2351 with its next release.