Use fetchcontent populate to fetch tfm

When we call FetchContent_MakeAvailable in Cmake it fetches the content
and calls CMake of TFM complains about the missing configuration of
TFM_TARGET so replace it with FetchContent_Populate to fetches tfm sources alone

Signed-off-by: Rajkumar Kanagaraj <rajkumar.kanagaraj@arm.com>