travis-ci: build Tegra boards

ARMv7 Tegra boards aren't currently covered by any other travis-ci jobs.
Add a new job to build them.

Signed-off-by: Stephen Warren <swarren@nvidia.com>
Reviewed-by: Tom Rini <trini@konsulko.com>
Reviewed-by: Heiko Schocher <hs@denx.de>

.travis.yml

@@ -155,6 +155,8 @@ matrix:
     - env:
         - BUILDMAN="siemens"
     - env:
+        - BUILDMAN="tegra -x aarch64"
+    - env:
         - BUILDMAN="ti"
     - env:
         - BUILDMAN="aarch64"