arm: tegra: define fdtfile option for distro boot

For booting via UEFI we need to define the fdtfile option so
bootefi has the option to load a fdtfile from disk. For arm64
the kernel dtb is located in a vendor directory so we define
that as nvidia for that architecture.

Signed-off-by: Peter Robinson <pbrobinson@gmail.com>
Signed-off-by: Tom Warren <twarren@nvidia.com>