optimize float complex FFT

1. To optimize FFT, the algorithm is changed. Bit reversal is removed and radix 8 is added.
2. After test, the optimized FFT show the best performance, so that the old implementations are removed.

The performance result is as follows:

toolchain: gcc 4.8 at -O2
omx fft's execute time is the base. The ratio is less, the performance is better.

panda board A9:
|     |16    |32    |64    |128   |256   |512   |1024  |2048  |4096  |
|Ne10 |84.27%|89.57%|85.63%|85.79%|87.89%|87.91%|83.51%|97.08%|92.68%|
|omx  |100%  |100%  |100%  |100%  |100%  |100%  |100%  |100%  |100%  |

nexus10 A15:
|     |16    |32    |64    |128   |256   |512   |1024  |2048  |4096  |
|Ne10 |84.88%|98.43%|89.46%|101.0%|99.24%|103.2%|93.80%|105.1%|97.44%|
|omx  |100%  |100%  |100%  |100%  |100%  |100%  |100%  |100%  |100%  |

Change-Id: I363ee1602f08532e566d3a5a4f3d7a99972a1283