flash_unlock_acr allows to unlock RUN_PD bit from FLASH_ACR register. Relock is done automatically
when writing 0 to RUN_PD, so no flash_lock_acr method.
Turns out, there's lots of common code for flash. Pull up prefetch
on/off to start with, as there's only a single bit name different.
Pull up the definitions of common API functions too, starting with
flash_set_ws. Even if the implementations are different, things that
meant to be the same, should be defined centrally.
