flash: EFM32 flash implementation

Limited (no page unprotect, no block writes) implementation of EFM32
flash support. Verified with EFM32 development kit and STLink V2 adapter
using SWD.

Change-Id: I3db2054d9aa628a1fe4814430425db3c9959c71c
Signed-off-by: Roman D <me@iamroman.org>
Reviewed-on: http://openocd.zylin.com/1106
Tested-by: jenkins
Reviewed-by: Spencer Oliver <spen@spen-soft.co.uk>