clk: imx: vf610: add suspend/resume support

The clock register are lost when enterying LPSTOPx, hence provide
suspend/resume functions restoring them. The clock gates get
restored by the individual driver, hence we do not need to restore
them here.

Signed-off-by: Stefan Agner <stefan@agner.ch>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>