i2c: imx: Switch to RUNTIME_PM_OPS()

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

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

This allows removing the __maybe_unused notation from the runtime
suspend/resume() functions.

Signed-off-by: Fabio Estevam <festevam@denx.de>
Acked-by: Oleksij Rempel <o.rempel@pengutronix.de>
Signed-off-by: Andi Shyti <andi.shyti@kernel.org>