* Make SDRAM test cache-aware for newer MCUs.
* Use the defined data bus width (instead of the fixed 8-bits).
* Allow optional failure on error with verbose error messages.
* Test speed is now inverted (test accepts exhaustive instead fast).
These functions enable SDRAM data retention in stop mode. Example usage,
in mpconfigboard.h:
#define MICROPY_BOARD_ENTER_STOP sdram_enter_low_power();
#define MICROPY_BOARD_LEAVE_STOP sdram_leave_low_power();
