clk: tegra: Don't enable already enabled PLLs

Initially Common Clock Framework isn't aware of the clock-enable status,
this results in enabling of clocks that were enabled by bootloader. This
is not a big deal for a regular clock-gates, but for PLL's it may have
some unpleasant consequences. Thus re-enabling PLLX (the main CPU parent
clock) may result in extra long period of PLL re-locking.

Acked-by: Peter De Schrijver <pdeschrijver@nvidia.com>
Signed-off-by: Dmitry Osipenko <digetx@gmail.com>
Signed-off-by: Stephen Boyd <sboyd@kernel.org>