tegra: Indicate that binman makes all three output files

Use GNU make pattern rules to indicate that a single run of binman
produces all three Tegra output files. The avoids make running binman
three times (perhaps in parallel) and those instances inteferring with
each other.

See http://patchwork.ozlabs.org/patch/944611/

 for the bug report.

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