ARM: new platform for Energy Micro's EFM32 Cortex-M3 SoCs

There are still some missing parts (e.g. board support, device trees),
but with these bits added on top of this patch I can successfully boot a
EFM32GG-DK3750 board that uses an EFM32GG990F1024.

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>