.travis.yml: reduce the load on [ppc64] GCC check-tcg

This seems to be timing out quite often and occasionally running out
of disk space. Relegate it to light duties.

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20200501111505.4225-5-alex.bennee@linaro.org>
1 file changed