efm32: remove duplicate part name decoding.

The probe and info methods had duplicate sections decoding family names
to generate a human friendly part name.  Extract this to a common
helper.

Change-Id: I4c6309d83c601e154b7c14ad9c15c53854ee1e98
Signed-off-by: Karl Palsson <karlp@tweak.net.au>
Reviewed-on: http://openocd.zylin.com/2932


Tested-by: jenkins
Reviewed-by: Andreas Färber <afaerber@suse.de>
Reviewed-by: Freddie Chopin <freddie.chopin@gmail.com>