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>

include/linux/dmaengine.h

@@ -329,7 +329,7 @@ enum dma_slave_buswidth {
  * @src_addr_width: this is the width in bytes of the source (RX)
  * register where DMA data shall be read. If the source
  * is memory this may be ignored depending on architecture.
- * Legal values: 1, 2, 4, 8.
+ * Legal values: 1, 2, 3, 4, 8, 16, 32, 64.
  * @dst_addr_width: same as src_addr_width but for destination
  * target (TX) mutatis mutandis.
  * @src_maxburst: the maximum number of words (note: words, as in