serial: efm32: properly namespace location property

While reviewing an i2c driver for efm32 that needs a similar property
Wolfram Sang pointed out that "location" is a too generic name for something
that is efm32 specific. So add an appropriate namespace and fall back to the
generic name in case of failure.

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