asm-generic: io: remove {read,write} string functions

The {read,write}s{b,w,l} functions are not defined across all
architectures and therefore shouldn't be used by portable drivers. We
should encourage driver writers to use the io{read,write}{8,16,32}_rep
functions instead.

This patch removes the {read,write} string functions for the generic IO
header as they have no place in a new architecture port.

Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Mike Frysinger <vapier@gentoo.org>
Cc: Ben Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Will Deacon <will.deacon@arm.com>