spi: Drop unused efm32 bus driver

Support for this machine was just removed, so drop the now unused spi
bus driver, too.

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Link: https://lore.kernel.org/r/20210114151630.128830-5-u.kleine-koenig@pengutronix.de


Signed-off-by: Mark Brown <broonie@kernel.org>