Add root CMakeLists.txt to better support FetchContent (#64)

CMake's `FetchContent_MakeAvailable()` treats the fetched repository as
a CMake project and automatically calls `subdirectory()` on it, if a
`CMakeLists.txt` exists in the root directory.

Signed-off-by: Lingkai Dong <lingkai.dong@arm.com>

Signed-off-by: Lingkai Dong <lingkai.dong@arm.com>