fixmap: Allow architecture overriding set_fixmap_io

For a realm guest it will be necessary to ensure IO mappings are shared
so that the VMM can emulate the device. The following patch will provide
an implementation of set_fixmap_io for arm64 setting the shared bit (if
in a realm).

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