arm64: Lock realm RAM in memory

RMM doesn't yet support paging protected memory pages. Thus the VMM
must pin the entire VM memory.

Use mlock2 to keep the realm pages pinned in memory once they are faulted
in. Use the MLOCK_ONFAULT flag to prevent pre-mapping the pages and
maintain some semblance of on demand-paging for a realm VM.

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