serial/efm32: parse location property

The non-dt probing allowed passing the location via platform data from
the beginning. So make up leeway for device tree probing.

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