tf-a-main: Change LAVA_RETRIES to 2

That has already been set such for l2 jobs, and we had successful build with
2 retries for tf-a-main too: https://ci.trustedfirmware.org/job/tf-a-main/860/
(a lone failure there is an unrelated physical board problem).

Signed-off-by: Paul Sokolovsky <paul.sokolovsky@linaro.org>
Change-Id: I2ce2574e582e3f4b1020219b24b210113d73c8db
1 file changed