asm-generic/io.h: add io{read,write}64 accessors

This will allow device drivers to consistently use io{read,write}XX
also for 64-bit accesses.

Acked-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Horia Geantă <horia.geanta@nxp.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>