ARM: tegra: combine duplicate Makefile rules

The rules to generate u-boot-{no,}dtb-tegra.bin were almost identical.
Combine them into a single paremeterized rule. This will allow the next
patch to edit a single rule, rather than being cut/paste twice.

Signed-off-by: Stephen Warren <swarren@nvidia.com>
Acked-by: Simon Glass <sjg@chromium.org>
Signed-off-by: Tom Warren <twarren@nvidia.com>