arm64: Add --measurement-algo command line option for a realm

Add the command line option to specify the algorithm that will be used
to create the cryptographic measurement of the realm. Valid options are
"sha256" and "sha512". The final measurement will be a hash using the
selected algorithm

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