async_tx, dmaengine: document channel allocation and api rework

"Wouldn't it be better if the dmaengine layer made sure it didn't pass
the same channel several times to a client?

I mean, you seem concerned that the memcpy() API should be transparent
and easy to use, but the whole registration interface is just
ridiculously complicated..."
	- Haavard

The dmaengine and async_tx registration/allocation interface is indeed
needlessly complicated.  This redesign has the following goals:

1/ Simplify reference counting: dma channels are not something one would
   expect to be hotplugged, it should be an exceptional event handled by
   drivers not something clients should be mandated to handle in a
   callback.  The common case channel removal event is 'rmmod <dma driver>',
   which for simplicity should be disallowed if the channel is in use.
2/ Add an interface for requesting exclusive access to a channel
   suitable to device-to-memory users.
3/ Convert all memory-to-memory users over to a common allocator,...