asm-generic/io.h: add reads[bwl]/writes[bwl] helpers

A bunch of arches define reads[bwl]/writes[bwl] helpers for accessing
memory mapped registers.  Since the Blackfin ones aren't specific to
Blackfin code, move them to the common asm-generic/io.h for people.

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