tf-a-tftf-gerrit-tforg-l1: Sync params with lts2.8 job

Add TF_GERRIT_BRANCH, TF_GERRIT_REFSPEC. Adjust param docstrings.

Signed-off-by: Paul Sokolovsky <paul.sokolovsky@linaro.org>
Change-Id: I4add0b06797d369fb953cb8cceef0fabd9181354
1 file changed