fvp_mps2.jinja2: Increase --cpulimit to 300 for coverage

Some jobs were spotted which exceeded older limit of 240, e.g.
https://tf.validation.linaro.org/scheduler/job/868138

Signed-off-by: Paul Sokolovsky <paul.sokolovsky@linaro.org>
Change-Id: I28dfdd911021ee7daf6df5a979b92bda17ff07fc
1 file changed