fvp-linux.rootfs+rst31: Extend timeout to 21min

Case:
https://ci.trustedfirmware.org/job/tf-a-builder/1657283/console
https://tf.validation.linaro.org/scheduler/job/1311439

Signed-off-by: Paul Sokolovsky <paul.sokolovsky@linaro.org>
Change-Id: I97f625d37ed9bc804041d9b41c0f405ddcea52ed
(cherry picked from commit 9dd496245c89efed6dd573340de358fbd4977149)
1 file changed