Docs: Add design document of multi-core memory access check

Describe the general check process, data types and APIs definitions
of multi-core memory access check.

This design document of multi-core memory access check has been
reviewed and accepted as patch [1] on feature-twincpu branch

[1] https://review.trustedfirmware.org/c/trusted-firmware-m/+/1601



Change-Id: I5fd1b9923d318faae772eeee1d5bbd3741128ef8
Signed-off-by: David Hu <david.hu@arm.com>