Add RP2350 cross-partition flash access support

[DavidMenting]

The RP2350 boot rom restricts access to flash addresses outside of the current partition when partitions are used. This change uses XIP_NOCACHE_NOALLOC_NOTRANSLATE_BASE on RP2350 to enable access to flash regions beyond the currently booted partition. On RP2040, continue using XIP_BASE for optimal cached performance.

On RP2350's without partitions this might result in slightly lower performance, although I suspect the XIP cache is not really useful when working with virtual filesystems anyway.

Fixes #65: RP2350 partitioned flash access restriction

[DavidMenting]

A discussion about this issue is here: https://forums.raspberrypi.com/viewtopic.php?p=2335484#p2335484

A possible improvement for futureproofing is changing the #define to check whether XIP_NOCACHE_NOALLOC_NOTRANSLATE_BASE is defined like so:

#ifdef XIP_NOCACHE_NOALLOC_NOTRANSLATE_BASE
    const uint8_t *flash_contents = (const uint8_t *)(XIP_NOCACHE_NOALLOC_NOTRANSLATE_BASE + flash_target_offset(device) + (size_t)addr);
#else
    const uint8_t *flash_contents = (const uint8_t *)(XIP_BASE + flash_target_offset(device) + (size_t)addr);
#endif

or, if we want to never read through the cache:

#ifdef XIP_NOCACHE_NOALLOC_NOTRANSLATE_BASE
    #define FLASH_BASE XIP_NOCACHE_NOALLOC_NOTRANSLATE_BASE
#else
    #define FLASH_BASE XIP_NOCACHE_NOALLOC_BASE
#endif

const uint8_t *flash_contents = (const uint8_t *)(FLASH_BASE + flash_target_offset(device) + (size_t)addr);