serial/imx: make devdata member point to const data

This is only cosmetic for now. In case that

	http://mid.gmane.org/1335171381-24869-1-git-send-email-u.kleine-koenig@pengutronix.de

will be applied, it fixes a warning

	drivers/tty/serial/imx.c: In function 'serial_imx_probe_dt':
	drivers/tty/serial/imx.c:1430:17: warning: assignment discards 'const' qualifier from pointer target type [enabled by default]

though.

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>