soc/tegra: pmc: Don't allocate struct tegra_powergate on stack

With a later commit an instance of the struct device will be added to
struct genpd and with that the size of the struct tegra_powergate will
be over 1024 bytes. That generates following warning:

drivers/soc/tegra/pmc.c:579:1: warning: the frame size of 1200 bytes is larger than 1024 bytes [-Wframe-larger-than=]

Avoid such warnings by allocating the structure dynamically.

Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>
Acked-by: Thierry Reding <treding@nvidia.com>