arm: fix array-out-of-bounds upon register parsing error

arm: fix array-out-of-bounds upon register parsing error

Despite the comment ahead of the enum explicitly pointing out the need
to also update the corresponding array, 1b8833198c0 ("Add support for
MVE instructions: vcmp and vpt") failed to do so. Oddly enough the issue
appears to be spotted only by rather old gcc (4.3-ish in my case).