rom_flash_exit_xip() function

Return the QSPI device from its XIP state to a serial command state \if rp2040_specific On RP2040, first set up the SSI for serial-mode operations, then issue the fixed XIP exit sequence described in Section 2.8.1.2 of the datasheet. Note that the bootrom code uses the IO forcing logic to drive the CS pin, which must be cleared before returning the SSI to XIP mode (e.g. by a call to _flash_flush_cache). This function configures the SSI with a fixed SCK clock divisor of /6. \endif \if rp2350_specific On RP2350, Initialise the QMI for serial operations (direct mode), and also initialise a basic XIP mode, where the QMI will perform 03h serial read commands at low speed (CLKDIV=12) in response to XIP reads. Then, issue a sequence to the QSPI device on chip select 0, designed to return it from continuous read mode ("XIP mode") and/or QPI mode to a state where it will accept serial commands. This is necessary after system reset to restore the QSPI device to a known state, because resetting RP2350 does not reset attached QSPI devices. It is also necessary when user code, having already performed some continuous-read-mode or QPI-mode accesses, wishes to return the QSPI device to a state where it will accept the serial erase and programming commands issued by the bootrom's flash access functions. If a GPIO for the secondary chip select is configured via FLASH_DEVINFO, then the XIP exit sequence is also issued to chip select 1. The QSPI device should be accessible for XIP reads after calling this function; the name flash_exit_xip refers to returning the QSPI device from its XIP state to a serial command state. \endif