ARM: tegra: Enable position independent build for 64-bit

ARM: tegra: Enable position independent build for 64-bit

Note that U-Boot is always chainloaded from cboot starting with L4T
release 28. cboot always loads U-Boot to a fixed address, so making
the builds position independent isn't strictly necessary. However,
position independent builds can be convenient because if U-Boot is
ever loaded to an address different from its link address, it will
still be able to boot.

Signed-off-by: Thierry Reding <treding@nvidia.com>
Signed-off-by: Tom Warren <twarren@nvidia.com>