cmake: stop FetchContent from fetching content

CMake includes a FetchContent module, which will download further
source code at configure time.  With the network isolation this will now
fail, but as not all environments support network isolation we can tell
cmake to not download either for extra safety.

Signed-off-by: Ross Burton <ross.burton@arm.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>