MLK-17089-4: ASoC: fsl_spdif: support suspend & resume for imx8

Base on latest power management design in MLK-17074, every driver
need to enter runtime suspend state in suspend, so the driver should
call the pm_runtime_force_suspend in suspend. with this implementation
the suspend function almost same as runtime suspend function. so remove
the suspend function, just use pm_runtime_force_suspend instead.

Signed-off-by: Shengjiu Wang <shengjiu.wang@nxp.com>