travis.yml: Add in uniphier as a job, modify aarch64 builds a bit

- Add in system aarch64-linux-gnu toolchain
- Now that all VMs will have aarch64 available, don't exclude them from
  other jobs but instead exclude them from the catch-all aarch64 build
- Add JOB= to the Freescale/ARM build to be clear about what it does.
- Add uniphier as a stand-alone job

Signed-off-by: Tom Rini <trini@konsulko.com>