soc/tegra: pmc: Ensure mutex is always initialised

soc/tegra: pmc: Ensure mutex is always initialised

The mutex used by the PMC driver may not be initialised if early
initialisation of the driver fails. If this does happen, then it could
be possible for callers of the public PMC functions to still attempt to
acquire the mutex. Fix this by initialising the mutex as soon as
possible to ensure it will always be initialised.

Signed-off-by: Jon Hunter <jonathanh@nvidia.com>
Signed-off-by: Thierry Reding <treding@nvidia.com>