Add a debug assertion in the initialization function of Trusty's SPD
to check for the presence of Trusty. If Trusty is absent then the SPD's
setup function already detects it and returns an error code so the init
function will never been called. Therefore, a debug assertion is enough
to catch this improbable error case.

Change-Id: Id20013e9291cdeef7827b919de2a22455f6cd9f9
Signed-off-by: Sandrine Bailleux <sandrine.bailleux@arm.com>