flash: efm32: Add support for EZR32LG and EZR32WG.

This patch was tested with an EZR32WG Starter Kit.

Change-Id: I0f7c619e715fe30e88e6da3bead0806dd3bce819
Signed-off-by: Marc Schink <openocd-dev@marcschink.de>
Reviewed-on: http://openocd.zylin.com/2700


Tested-by: jenkins
Reviewed-by: Spencer Oliver <spen@spen-soft.co.uk>