cpu/cortexm: pointer to .text should be const
cpu/cortexm: pointer to .text should be const

A pointer to read-only ROM data should be const.

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