tegra: Define CONFIG_SKIP_LOWLEVEL_INIT for SPL build

Then we can get rid of the #ifdef CONFIG_TEGRA guard in cpu_init_crit.

Signed-off-by: Axel Lin <axel.lin@ingics.com>
Tested-by: Stephen Warren <swarren@nvidia.com>
Signed-off-by: Tom Warren <twarren@nvidia.com>