arm: realm: Add a test for shared memory

Do some basic tests that trigger marking a memory region as
RIPAS_EMPTY and accessing the shared memory. Also, convert it back
to RAM and make sure the contents are scrubbed.

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