cros: Disable coreboot related BL31 features

I don't know why, but removing this BL31 make argument lets gru-kevin
power off properly when shut down from Linux. Needs investigation.
Do it as a cros-only HACK patch so people don't have to hold the power
button after every shutdown.

Signed-off-by: Alper Nebi Yasak <alpernebiyasak@gmail.com>