serial/mpc52xx_uart: add a const qualifier

This prepares *of_device_id.data becoming const. Without this change
the following warning would occur:

	drivers/tty/serial/mpc52xx_uart.c: In function 'mpc52xx_uart_of_enumerate':
	drivers/tty/serial/mpc52xx_uart.c:1440:11: warning: assignment discards 'const' qualifier from pointer target type [enabled by default]
Acked-by: NGreg Kroah-Hartman <gregkh@linuxfoundation.org>
Acked-by: NBenjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: NUwe Kleine-König <u.kleine-koenig@pengutronix.de>

drivers/tty/serial/mpc52xx_uart.c

@@ -598,7 +598,7 @@ static struct psc_ops mpc512x_psc_ops = {
 };
 #endif
 
-static struct psc_ops *psc_ops;
+static const struct psc_ops *psc_ops;
 
 /* ======================================================================== */
 /* UART operations                                                          */