soc/samsung/exynos: Make 'ps_hold_setup()' static

Change-Id: I272fea9c2767c341e8a545bf7a9ac18eefa2bda5
Signed-off-by: Edward O'Callaghan <eocallaghan@alterapraxis.com>
Reviewed-on: http://review.coreboot.org/7917
Tested-by: build bot (Jenkins)
Reviewed-by: David Hendricks <dhendrix@chromium.org>