Replace deprecated CMake function call

FetchContent_Populate has been deprecated in upstream CMake. Replace the
usage with FetchContent_MakeAvailable. Upstream documentation currently
states that it's possible to replicate the Populate behaviour when using
MakeAvailable, however it's not the case. Add an invalid path to
SOURCE_SUBDIR, forcing CMake based subprojects not to be configured or
built.

Change-Id: I0cf1fd8ee7a374bc793d1763bd2d2167e7d9ad2c
Signed-off-by: Jonny Svärd <jonny.svaerd@arm.com>