serial: mpc52xx_uart: Remove legacy PM hook

The legacy PM hook was never implemented. If we would like to achieve this,
the entire serial subsystem should switch to use runtime PM first. For now,
remove unneeded code.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Link: https://lore.kernel.org/r/20250117141304.592611-1-andriy.shevchenko@linux.intel.com


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