tegra: define fdt_load/fdt_high variables

These variables act like loadaddr, but for a device tree image. Defining
them in the environment allows boot scripts to be identical on Tegra20 and
Tegra30, which have different memory base addresses, and hence need
different values for these variables.

Signed-off-by: Stephen Warren <swarren@nvidia.com>
Signed-off-by: Tom Warren <twarren@nvidia.com>