samd: audio_dma: Track channel allocation

Previously, we depended on allocated channels to always be
"dma_channel_enabled".  However, (A) sometimes, many operations
would take place between find_free_audio_dma_channel and
audio_dma_enable_channel, and (B) some debugging I did led me to believe
that "dma_channel_enabled" would become false when the hardware ended
a scheduled DMA transaction, but while a CP object would still think it
owned the DMA channel.

((B) is not documented in the datasheet and I am not 100% convinced that
my debugging session was not simply missing where we were disabling the
channel, but in either case, it shows a need to directly track allocated
separately from enabled)

Therefore,
 * Add audio_dma_{allocate,free}_channel.
   * audio_dma_free_channel implies audio_dma_disable_channel
   * track via a new array audio_dma_allocated[]
 * clear all allocated flags on soft-reboot
 * Convert find_free_audio_dma_channel to audio_dma_allocate_channel
   * use audio_dma_allocated[] instead of dma_channel_enabled() to check
     availability
 * remove find_free_audio_dma_channel
 * For each one, find a matching audio_dma_disable_channel to convert
   to audio_dma_free_channel

Closes: #2058

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