travis: use mpc85xx-p2020 sdk instead of ar71xx

travis: use mpc85xx-p2020 sdk instead of ar71xx

Target ar71xx is deprecated and removed in the master branch and makes SDK
not available anymore. Travis fails because of that.
It was superseded by target ath79. These devices have 4 MB flash and/or 32 MB RAM.

However, ath79 is being used by CircleCI if you have it configured for
your repository and if you are contributing to this repository. It
is not good to have two CI for the same target. Let's use powerpc.

Signed-off-by: Josef Schlehofer <pepe.schlehofer@gmail.com>
(cherry picked from commit 4526fb3eae6ed5fdbc5f6cf64a56b4471b54f9d0)