ARM: tegra: cpufreq: move clk_get/put out of function tegra_cpu_init/exit

tegra_cpu_init/exit will be called every time one cpu core is online or
offline. And all cpu cores share same clocks, redundant clk_get/put
wast time,  so I move them out.

Signed-off-by: Richard Zhao <linuxzsc@gmail.com>
Signed-off-by: Stephen Warren <swarren@nvidia.com>