asm-generic/io.h: allow people to override individual funcs

For the Blackfin port, we can use much of the asm-generic/io.h header,
but we still need to declare some of our own versions of functions.
Like the __raw_read* and in/out "string" helpers.  So let people do
this easily for many of these funcs.

Signed-off-by: Mike Frysinger <vapier@gentoo.org>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>