arm: mm: Add export symbol for set_memory_* functions

Modules may need to use the set_memory_* functions if they
explicitly need to have self modifying code or other features
that rely on changing memory that normally shouldn't be changed.
Match other architectures and add the exports.

Change-Id: Ifa473d6d57ce113227fa4cd6bd25f7b18bd1cca0
Signed-off-by: Laura Abbott <lauraa@codeaurora.org>