spi: 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: Mark Brown <broonie@linaro.org>