uboot-imx

.travis.yml: Further optimizations

- Xilinx aarch64 is caught in the general xilinx arm job, exclude from
  the general aarch64 job.
- Give the generic aarch64 job a better name
- Re-sort the PowerPC jobs so that we can complete them a bit quicker.

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