rom_flash_flush_cache() function

Flush the XIP cache \if rp2040_specific Flush and enable the XIP cache. Also clears the IO forcing on QSPI CSn, so that the SSI can drive the flash chip select as normal. \endif \if rp2350_specific Flush the entire XIP cache, by issuing an invalidate by set/way maintenance operation to every cache line. This ensures that flash program/erase operations are visible to subsequent cached XIP reads. Note that this unpins pinned cache lines, which may interfere with cache-as-SRAM use of the XIP cache. No other operations are performed. \endif