serial: efm32: correct namespacing of location property

Olof Johansson pointed out that usually the company name is picked as
namespace prefix to specific properties. So expect "energymicro,location"
but fall back to the previously introduced name "efm32,location".

Cc: Olof Johansson <olof@lixom.net>
Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>