ARM: tegra: cpuidle: use firmware for power down

Attempt to invoke the prepare_idle() and do_idle() firmware calls
to power down a CPU so an underlying firmware gets informed of
the idle operation and performs it by itself if designed in such a way.

Signed-off-by: Alexandre Courbot <acourbot@nvidia.com>
Acked-by: Olof Johansson <olof@lixom.net>
Signed-off-by: Stephen Warren <swarren@nvidia.com>