soc/tegra: pmc: Prevent racing with cpuilde driver

Both PMC and cpuidle drivers are probed at the same init level and
cpuidle depends on the PMC suspend mode. Add new default suspend mode
that indicates whether PMC driver has been probed and reset the mode in
a case of deferred probe of the PMC driver.

Signed-off-by: Dmitry Osipenko <digetx@gmail.com>
Signed-off-by: Thierry Reding <treding@nvidia.com>