cpu/efm32: remove ADC defintion for CPUs without ADC
cpu/efm32: remove ADC defintion for CPUs without ADC

Gecko Series 2 is shipped with a new IADC peripheral which is backed by its own emlib driver.

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