tegra: Init clocks even when SPL did not run

At present early clock init happens in SPL. If SPL did not run (because
for example U-Boot is chain-loaded from another boot loader) then the
clocks are not set as U-Boot expects.

Add a function to detect this and call the early clock init in U-Boot
proper.

Signed-off-by: Simon Glass <sjg@chromium.org>