Fix for #4292
This will let wait_for_safe_mode_reset run if the reset reason is RESET_REASON_UNKNOWN to account for ports that don't implement it fully.

However it might not make sense when the reset reason is implemented though, since we only want to do that when powering on or pressing reset. So a better fix might be to use a #define (in each port's Processor.c/h) to remove the test altogether when not fully implemented (that might even save a few bytes on those ports).

Ultimately the fix is to fully implement common_hal_mcu_processor_get_reset_reason for each port, but I'll leave that as an exercise to the reader.