clk: tegra30: Use Tegra CPU powergate helper function

Rather than using the tegra_powergate_is_powered() function for
determining if a CPU is powered, use the tegra_pmc_cpu_is_powered()
instead which was created to get the CPU power status. Internally
tegra_pmc_cpu_is_powered() calls tegra_powergate_is_powered() and so
is equivalent.

The Tegra30 clock driver is the only public user of
tegra_powergate_is_powered() and so by updating the Tegra30 clock
driver to use tegra_pmc_cpu_is_powered(), we can then make
tegra_powergate_is_powered() a non-public function.

Signed-off-by: Jon Hunter <jonathanh@nvidia.com>
Acked-by: Thierry Reding <treding@nvidia.com>
Signed-off-by: Stephen Boyd <sboyd@kernel.org>