dma: Add max block count attribute

Adds an attribute that describes the maximum number of scatter gather
blocks that the DMA will accept. Useful in cases where DMA is used as
the abstraction for moving data around (e.g. SoF audio streams) and
variances of the hardware need to inform decisions about how to use
the DMA API.

Co-authored-by: Adrian Bonislawski <adrian.bonislawski@intel.com>
Co-authored-by: Jaska Uimonen <jaska.uimonen@intel.com>
Signed-off-by: Tom Burdick <thomas.burdick@intel.com>