cpu/efm32: keep debug unit active during EM2 when DEVELHELP is active
cpu/efm32: keep debug unit active during EM2 when DEVELHELP is active

ant9000/RIOT,ant9000/RIOT,RIOT-OS/RIOT,kaspar030/RIOT,ant9000/RIOT,jasonatran/RIOT,kaspar030/RIOT,jasonatran/RIOT,jasonatran/RIOT,RIOT-OS/RIOT,kaspar030/RIOT,RIOT-OS/RIOT,RIOT-OS/RIOT,RIOT-OS/RIOT,kaspar030/RIOT,ant9000/RIOT,jasonatran/RIOT,kaspar030/RIOT,ant9000/RIOT,jasonatran/RIOT