Add GBB_FLAG_DISABLE_FWMP documentation to set_gbb_flags.sh

Looks like we forgot to update the help text for set_gbb_flags.sh when
adding the DISABLE_FMWP flag. This patch fixes that.

BRANCH=None
BUG=None
TEST=None

Change-Id: Iae45e151ae786565f6a1a695a2e3c3d01f8c1d0a
Signed-off-by: Julius Werner <jwerner@chromium.org>
Reviewed-on: https://chromium-review.googlesource.com/976801
Reviewed-by: Randall Spangler <rspangler@chromium.org>