All the "f" type flash parts have an EOP flag, even if it's in different
bit positions. Add a header for this common functionality, and move
it's implementation to the existing common file.
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.
