rom_flash_enter_cmd_xip() function

Configure the SSI/QMI with a standard command Configure the SSI/QMI to generate a standard 03h serial read command, with 24 address bits, upon each XIP access. This is a slow XIP configuration, but is widely supported. CLKDIV is set to 12 on RP2350. The debugger may call this function to ensure that flash is readable following a program/erase operation. Note that the same setup is performed by flash_exit_xip(), and the RP2350 flash program/erase functions do not leave XIP in an inaccessible state, so calls to this function are largely redundant on RP2350. It is provided on RP2350 for compatibility with RP2040.