travis: Enable OvS Travis CI for arm

travis: Enable OvS Travis CI for arm

Enable part of travis jobs with gcc compiler for arm64 architecture

1. Add arm jobs into the matrix in .travis.yml configuration file
2. To enable OVS-DPDK jobs, set the build target according to
different CPU architectures
3. Temporarily disable sparse checker because of static code checking
failure on arm64

Considering the balance of the CI coverage and running time, some kernel
and DPDK jobs are removed from Arm CI.

Successful travis build jobs report:
https://travis-ci.org/github/yzyuestc/ovs/builds/666129448

Reviewed-by: Yanqin Wei <Yanqin.Wei@arm.com>
Reviewed-by: Ruifeng Wang <Ruifeng.Wang@arm.com>
Reviewed-by: JingZhao Ni <JingZhao.Ni@arm.com>
Reviewed-by: Gavin Hu <Gavin.Hu@arm.com>
Signed-off-by: Lance Yang <Lance.Yang@arm.com>
Signed-off-by: Ilya Maximets <i.maximets@ovn.org>