periph/spi: add doc on low power implications
periph/spi: add doc on low power implications

yogo1212/RIOT,yogo1212/RIOT,OTAkeys/RIOT,ant9000/RIOT,basilfx/RIOT,x3ro/RIOT,aeneby/RIOT,jasonatran/RIOT,OTAkeys/RIOT,yogo1212/RIOT,OTAkeys/RIOT,smlng/RIOT,jasonatran/RIOT,aeneby/RIOT,basilfx/RIOT,jasonatran/RIOT,kaspar030/RIOT,smlng/RIOT,basilfx/RIOT,mtausig/RIOT,ant9000/RIOT,x3ro/RIOT,kaspar030/RIOT,josephnoir/RIOT,authmillenon/RIOT,yogo1212/RIOT,authmillenon/RIOT,miri64/RIOT,basilfx/RIOT,smlng/RIOT,josephnoir/RIOT,ant9000/RIOT,miri64/RIOT,kYc0o/RIOT,mtausig/RIOT,miri64/RIOT,aeneby/RIOT,jasonatran/RIOT,kYc0o/RIOT,kYc0o/RIOT,yogo1212/RIOT,authmillenon/RIOT,x3ro/RIOT,OlegHahm/RIOT,authmillenon/RIOT,smlng/RIOT,kYc0o/RIOT,RIOT-OS/RIOT,mtausig/RIOT,RIOT-OS/RIOT,josephnoir/RIOT,authmillenon/RIOT,basilfx/RIOT,mtausig/RIOT,josephnoir/RIOT,toonst/RIOT,OlegHahm/RIOT,OlegHahm/RIOT,x3ro/RIOT,yogo1212/RIOT,josephnoir/RIOT,aeneby/RIOT,OTAkeys/RIOT,authmillenon/RIOT,kaspar030/RIOT,kaspar030/RIOT,RIOT-OS/RIOT,OTAkeys/RIOT,kaspar030/RIOT,toonst/RIOT,miri64/RIOT,OlegHahm/RIOT,aeneby/RIOT,miri64/RIOT,ant9000/RIOT,toonst/RIOT,OlegHahm/RIOT,toonst/RIOT,kYc0o/RIOT,x3ro/RIOT,smlng/RIOT,mtausig/RIOT,toonst/RIOT,jasonatran/RIOT,RIOT-OS/RIOT,ant9000/RIOT,RIOT-OS/RIOT