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: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Acked-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>