travis-ci: Build mvebu boards (arm & aarch64) in separate job

Its easier to watch the output of the build process when the platforms
specific boards are grouped in a separate job. This patch adds a job
for all mvebu boards (arm and aarch64).

Signed-off-by: Stefan Roese <sr@denx.de>
Cc: Tom Rini <trini@konsulko.com>
Reviewed-by: Tom Rini <trini@konsulko.com>