exynos: Enable PSHOLD in SPL

There is quite a tight deadline in enabling PSHOLD, less than a second.
In some cases (e.g. with USB download), U-Boot takes longer than that
to load, so the board powers off before U-Boot starts.

Add a call in SPL to enable PSHOLD.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Lukasz Majewski <l.majewski@samsung.com>
Signed-off-by: Minkyu Kang <mk7.kang@samsung.com>