Support for debug interface lock of EFM32 controllers

Support for debug interface lock of EFM32 controllers

The capability to lock the debug interface on EFM32
controllers was lacking in OpenOCD.
After receiving some pointers by zapb_ and PaulFertser
on IRC (thanks guys!) I have added this capability.

This works by writing the required bits in the debug
lock word to '0'.

Note: there is currently no way to re-enable the debug
interface from OpenOCD as doing this requires specific
pin wiggling that is currently not implemented yet.

However: having the capability to lock the debug interface
is useful when building a volume programming jig.
You can flash the program code, verify and then
lock the debug interface so that the device cannot
be read when it is deployed in the field.

Change-Id: If2d562dfdb4b95519785a4395f755d9ae3d0cf12
Signed-off-by: Lieven Hollevoet <hollie@lika.be>
Reviewed-on: http://openocd.zylin.com/3389
Tested-by: jenkins
Reviewed-by: Freddie Chopin <freddie.chopin@gmail.com>