mtd: mtk-nor: add suspend/resume support

Abstract functions of clock setting, to avoid duplicated code,
these functions been used in new feature.
Implement suspend/resume functions.

Signed-off-by: Guochun Mao <guochun.mao@mediatek.com>
Signed-off-by: Cyrille Pitchen <cyrille.pitchen@wedev4u.fr>