ARM: efm32: select AUTO_ZRELADDR

The efm32 platform does not provide a zreladdr-y line its Makefile.boot,
so we always have to use CONFIG_AUTO_ZRELADDR in order to successfully
build and link a zImage.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Cc: Uwe Kleine-König <kernel@pengutronix.de>