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
2 files changed