optee: Move optee files into a new parent directory

Directory structure clean-up for better differentiation of Arm and
non-Arm developed code. Optee files are moved into a common parent
directory under product/.

unit_test: Remove optee/mbx from unit_test/CMakeLists.txt list of
modules because it's unit test is moved to product/. optee/mbx unit
test will be added back under product/ in another new patch.

Signed-off-by: Wen Ping Teh <wenping.teh@arm.com>
Change-Id: I9440ca518dee25bc73c09b863f32c65f4a858675