arm64: Mark all I/O as non-secure shared

All I/O is by default considered non-secure for realms. As such
mark them as shared with the host.

Co-developed-by: Suzuki K Poulose <suzuki.poulose@arm.com>
Signed-off-by: Suzuki K Poulose <suzuki.poulose@arm.com>
Signed-off-by: Steven Price <steven.price@arm.com>
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Changes since v3:
 * Add PROT_NS_SHARED to FIXMAP_PAGE_IO rather than overriding
   set_fixmap_io() with a custom function.
 * Modify ioreamp_cache() to specify PROT_NS_SHARED too.