[usbdev] Test operation at frequency corners

### Description

With the chip-level usb suspend-resume test the DPI model has ended up operating on a separate clock in order that it remains operable whilst the chip is powered down. (Local branch at present)

This permits us to try quickly a simple experiment with the usbdev and host (DPI) at opposite frequency extremes to see whether the device can still correctly receive maximum-length packets, and perhaps whether calibrating the 48MHz usbclk operates as intended using SOF packets.

@a-will for awareness; you have often mentioned this, and the chip-level suspend-resume is not yet ready to create a PR

Estimation: 4 for the initial experiment at frequency extremes; 12 for both that and oscillator calibration (reading, learning and experimentation required)

@johngt

> estimate 12
> remaining 2023-06-26 4