soc/tegra: pmc: move to using a restart handler

The pmc driver was previously exporting tegra_pmc_restart, which was
assigned to machine_desc.init_machine, taking precedence over the
restart handlers registered through register_restart_handler().

Signed-off-by: David Riley <davidriley@chromium.org>
[tomeu.vizoso@collabora.com: Rebased]
Signed-off-by: Tomeu Vizoso <tomeu.vizoso@collabora.com>
Acked-by: Stephen Warren <swarren@nvidia.com>
Reviewed-by: Alexandre Courbot <acourbot@nvidia.com>
[treding@nvidia.com: minor cleanups]
Signed-off-by: Thierry Reding <treding@nvidia.com>