arm: realm: Enable memory encryption

Enable memory encryption support for Realms.

When a page is "decrypted", we set the RIPAS to EMPTY, hinting to the hypervisor
that it could reclaim the page backing the IPA. Also the pagetable is updated
with the PTE_NS_SHARED attrbiute, which in effect turns the "ipa" to the
unprotected alias.

Similarly for "encryption" we mark the IPA back to RIPAS_RAM and clear the
PTE_NS_SHARED attribute.

The addresses passed into the helpers must be idmap/linear map addresses.

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