dmaengine: List all allowed values for src/dst_addr_width in kernel doc

Commit 93c6ee94 ("dma: Support for 3 bytes word size") and
commit 534a7298 ("dmaengine: Add 16 bytes, 32 bytes and 64 bytes
bus widths") added additional values for the allowed word size, but
omitted these from the struct dma_slave_config documentation.

Signed-off-by: Stefan Brüns <stefan.bruens@rwth-aachen.de>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>