clk: imx: imx8mp: Switch to RUNTIME_PM_OPS()

Replace SET_RUNTIME_PM_OPS() with its modern alternative RUNTIME_PM_OPS().

The combined usage of pm_ptr() and RUNTIME_PM_OPS() allows the
compiler to evaluate if the suspend/resume() functions are used at
buid time or are simply dead code.

This fixes the following s390 allmodconfig build errors:

drivers/clk/imx/clk-imx8mp-audiomix.c:363:12: error:
'clk_imx8mp_audiomix_runtime_resume' defined but not used
[-Werror=unused-function]
  363 | static int clk_imx8mp_audiomix_runtime_resume(struct device *dev)
      |            ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
drivers/clk/imx/clk-imx8mp-audiomix.c:356:12: error:
'clk_imx8mp_audiomix_runtime_suspend' defined but not used
[-Werror=unused-function]
  356 | static int clk_imx8mp_audiomix_runtime_suspend(struct device *dev)
      |            ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
cc1: all warnings being treated as errors

Reported-by: Naresh Kamboju <naresh.kamboju@linaro.org>
Closes: https://lore.kernel.org/linux-clk/CA+G9fYuP7S+a89Ep5g5_Ad69EMwRkJ8nM+MMTzbEcP+6H2oMXQ@mail.gmail.com/T/#u
Fixes: 1496dd41

 ("clk: imx: imx8mp: Add pm_runtime support for power saving")
Signed-off-by: Fabio Estevam <festevam@denx.de>
Reviewed-by: Peng Fan <peng.fan@nxp.com>
Link: https://lore.kernel.org/r/20240429214502.1363592-1-festevam@gmail.com
Signed-off-by: Abel Vesa <abel.vesa@linaro.org>