hardware_xosc: Hardcoded frequency range in xosc_init() prevents using >15MHz crystals on RP2350 without modifying the SDK

[Vesek]

When trying to configure pico-sdk to use a 32MHz crystal as the clock source for XOSC I found that xosc_init() in xosc.c hardcodes the XOSC CTRL register to XOSC_CTRL_FREQ_RANGE_VALUE_1_15MHZ. This cannot be overridden without modifying the SDK code.

It would be incredibly helpful if the frequency range in xosc_init() could be made configurable via a preprocessor definition, similar to other clock configurations in the SDK.

For example, introducing a define like:

#ifndef PICO_XOSC_FREQ_RANGE
#define PICO_XOSC_FREQ_RANGE XOSC_CTRL_FREQ_RANGE_VALUE_1_15MHZ
#endif

so developers can override this behavior in their board.h files.

[lurch]

As an aside, it looks like the // Assumes 1-15 MHz input, checked above. comment inside xosc_init() is also wrong! Seems like da47661 changed the "check above" from #if XOSC_MHZ < 1 || XOSC_MHZ > 15 to #if XOSC_MHZ < 1 || XOSC_MHZ > 50, but then forgot to update the comment inside xosc_init() to match.

RP2040 could only use crystals up to 15MHz, which is likely why this XOSC_CTRL_FREQ_RANGE_VALUE_1_15MHZ value was originally hardcoded, but then RP2350 upped the maximum crystal frequency to 50MHz.