Exynos: Make sure ps_hold gets set in the SPL

Setting ps_hold ought to be one of the first things we do when we
first boot up. If we wait until the main u-boot runs we won't set it
in time and the PMIC may power us back off.

Signed-off-by: Doug Anderson <dianders@chromium.org>
Signed-off-by: Akshay Saraswat <akshay.s@samsung.com>
Acked-by: Simon Glass <sjg@chromium.org>
Signed-off-by: Minkyu Kang <mk7.kang@samsung.com>