powerpc: reserve two DMA channels for audio in MPC8610 HPCD device tree

The Freescale Elo DMA driver binds to all DMA channels in the device tree that
are compatible with "fsl,eloplus-dma-channel".  This conflicts with the sound
drivers for the MPC8610 HPCD.  On this board, the SSI uses two DMA channels and
therefore those channels are not available for general purpose use.  We
change the compatible properties for these channels "fsl,ssi-dma-channel".
This works because the sound drivers don't actually check the compatible
property when it grabs channels.

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