Add Support for DSP Block Processing

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Currently we use single sample processing.

Update Blackfin SPORT (I2S) peripheral driver with ping-pong buffers using DMA linked descriptors.

Add support for block processing in audio module callback and Aleph-DSP library.

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3070472

Sets up the SPORT DMA in linked descriptor mode.

Still uses single sample processing, copying the frame in the ISR.

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13b0240

Uses linked descriptors to fill 2 buffers and swaps pointers in the ISR. Still using single cycle processing.

I think this ping-pongs correctly, but it's hard to tell without JTAG. It sounds like it's working.

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6ffa796

Gets a block of interleaved samples via DMA; block size set in per_sport.h.

Monosynth example processes a block in a loop at the top level `module_process' function; Aleph DSP still uses single sample processing.

For block processing, we need to move the loops as low in the chain as possible; each Aleph DSP unit generator needs a *_next_block function, looping as little as possible.

We could also use 2D DMA to de-interleave the audio channels, so we can address each channel as a separate array.

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cf31c4c

Uses 2D triple buffered ping-pong DMA to automatically de-interleave audio channels.

Reports cycles used by module_process and cycles between I2S interrupts.

Aleph DSP makes some attempt at block processing, but dynamically allocates a lot to achieve per sample parameter interpolation; with a small enough block size we probably don't need this.

There are some discontinuities audible in the square wave. I thought it was an issue with the DMA timing, but it appears to also happen with single sample processing. Need to hook up a scope to check if it is happening with other waveforms, just not clearly audible.