soc/tegra: pmc: Add driver state syncing

Add driver state syncing that is invoked once all PMC consumers are
attached and ready. The consumers are the power domain clients.
The synchronization callback is invoked once all client drivers are
probed, the driver core handles this for us. This callback informs
PMC driver that all voltage votes are initialized by each PD client
and it's safe to begin voltage scaling of the core power domain.

Signed-off-by: Dmitry Osipenko <digetx@gmail.com>
Reviewed-by: Ulf Hansson <ulf.hansson@linaro.org>
[treding@nvidia.com: squash DT backwards-compatibility patch]
Signed-off-by: Thierry Reding <treding@nvidia.com>