dcadsp: Add a new method, qmf_32_subbands

This does most of the work formerly carried out by
the static function qmf_32_subbands() in dcadec.c.
arm: Add VFP-accelerated version of qmf_32_subbands

               Before           After
               Mean    StdDev   Mean    StdDev  Change
This function   1323.0  98.0      746.2  60.6   +77.3%
Overall        15400.0 336.4    14147.5 288.4    +8.9%