lib: sm: Add TRUSTY_FFA_MSG_IS_IDLE direct message Add a new FF-A direct message to check whether Trusty is idle on the current CPU. This is used by the bootloader to determine whether it should call into Trusty again with FFA_RUN or not. Bug: 28405701 Change-Id: If6f3f11716dfe6397c356b60833a0c05381c0220