powerpc: add SSI-to-DMA properties to Freescale MPC8610 HPCD device tree

Add the fsl,playback-dma and fsl,capture-dma properties to the Freescale
MPC8610 HPCD device tree.  These properties connect the SSI nodes to the
DMA nodes for the DMA channels that the SSI should use.  Also update the
ssi.txt documentation.

These properties will be needed when the ASoC V2 version of the Freescale
MPC8610 device drivers are merged into the mainline.

Signed-off-by: Timur Tabi <timur@freescale.com>
Signed-off-by: Kumar Gala <galak@kernel.crashing.org>