soc/tegra: pmc: Store PMC context in clocks

soc/tegra: pmc: Store PMC context in clocks

Clocks exposed by the PMC need to reference the PMC context for register
programming. Store a reference to the context in the data structure for
each clock to avoid the need for a global variable.

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