spi: efm32: Update binding document to make "efm32,location" property optional

If "efm32,location" property is not provided, keeping what is already
configured in the hardware, so its either the reset default 0 or whatever
the bootloader did.

Signed-off-by: Axel Lin <axel.lin@ingics.com>
Acked-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Mark Brown <broonie@linaro.org>