soc/tegra: pmc: Use PMC context embedded in powergates

soc/tegra: pmc: Use PMC context embedded in powergates

The powergates exposed by the PMC have a pointer to the PMC context
embedded. Use that embedded reference instead of relying on a global
variable.

For the core power domain a new structure needs to be introduced to wrap
the generic PM domain and store the PMC context.

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