ARM: trusted_foundations: Make prepare_idle call to take mode argument

The Trusted Foundations firmware call varies depending on the required
suspend-mode. Make the firmware API to take the mode argument in order
to expose all of the modes to firmware user.

Tested-by: Robert Yang <decatf@gmail.com>
Tested-by: Michał Mirosław <mirq-linux@rere.qmqm.pl>
Signed-off-by: Dmitry Osipenko <digetx@gmail.com>
Signed-off-by: Thierry Reding <treding@nvidia.com>