tegra: allow build to succeed with SPL disabled

u-boot-dtb-tegra.bin and u-boot-nodtb-tegra.bin binaries
are generated only if the SPL build is enabled as they have
dependency on SPL build

Signed-off-by: Vidya Sagar <vidyas@nvidia.com>
Signed-off-by: Tom Warren <twarren@nvidia.com>