ANDROID: KVM: arm64: Pin FFA mailboxes shared by the host

Gulshan reports that the hypervisor is not pinning the host FFA mailbox
pages, therefore allowing the host to unshare them after registration
and to later donate them for things like page-table pages.

Pin the host FFA mailboxes to prevent the host from unsharing them while
they are in use.

Bug: 228931886
Reported-by: Gulshan Singh <gsgx@google.com>
Signed-off-by: Will Deacon <willdeacon@google.com>
Change-Id: I18ecad6ccaa3ef89015a71d97890fad55f0568f2