arm64: Override set_fixmap_io

Override the set_fixmap_io to set shared permission for the host
in case of a CC guest. For now we mark it shared unconditionally.
Future changes could filter the physical address and make the
decision accordingly.

Signed-off-by: Suzuki K Poulose <suzuki.poulose@arm.com>
Signed-off-by: Steven Price <steven.price@arm.com>