arm: Introduce NS_SHARED PTE attribute

Introduce a new attribute to indicate the mapping is "Shared" with the
host. This will be used by the Realms to share pages with the Host.
For normal VMs, this is always 0.

For realms, this is dynamic, depending on the IPA width. The top bit of the
IPA is "treated" as the "NS_SHARED" attribute, making the VM access the
unprotected alias of the IPA.

By default, apply the NS_SHARED attribute for all I/O.

Signed-off-by: Joey Gouly <joey.gouly@arm.com>