clk: imx: imx8mp: Add pm_runtime support for power saving

Add pm_runtime support for power saving. In pm runtime suspend
state the registers will be reseted, so add registers save
in pm runtime suspend and restore them in pm runtime resume.

Signed-off-by: Shengjiu Wang <shengjiu.wang@nxp.com>
Reviewed-by: Peng Fan <peng.fan@nxp.com>
Reviewed-by: Marc Kleine-Budde <mkl@pengutronix.de>
Link: https://lore.kernel.org/r/1711026842-7268-1-git-send-email-shengjiu.wang@nxp.com
Signed-off-by: Abel Vesa <abel.vesa@linaro.org>