mfd: sec-core: Prepare regulators for suspend state to reduce power-consumption

This patch use regulator_suspend_prepare() function to prepare the proper state
of regulators for suspend state to remove un-necessary leakage power-consumption.

Signed-off-by: Chanwoo Choi <cw00.choi@samsung.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>