powerpc: 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: Michael Ellerman <mpe@ellerman.id.au>
Signed-off-by: Horia Geantă <horia.geanta@nxp.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>