tty: serial: fsl_lpuart: Use __maybe_unused instead of #if CONFIG_PM_SLEEP

Use __maybe_unused for power management related functions to simplify
the code.

Signed-off-by: Anson Huang <Anson.Huang@nxp.com>
Link: https://lore.kernel.org/r/1589547481-25932-1-git-send-email-Anson.Huang@nxp.com


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>