arm64: add io{read,write}64be accessors

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

Acked-by: Catalin Marinas <catalin.marinas@arm.com>
Signed-off-by: Alex Porosanu <alexandru.porosanu@nxp.com>
Signed-off-by: Horia Geantă <horia.geanta@nxp.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>