lib: sm: Add TRUSTY_FFA_MSG_IS_IDLE direct message am: 69127da74b

Original change: https://android-review.googlesource.com/c/trusty/lk/trusty/+/3453135

Change-Id: Ic797602c4cb4d68a17235a852d53c7dc397d8b05
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>