ASoC: MPC5200: replace of_device with platform_device

'struct of_device' no longer exists, and its functionality has been merged
into platform_device.  Update the MPC5200 audio DMA driver (mpc5200_dma)
accordingly.  This fixes a build break.

Signed-off-by: Timur Tabi <timur@freescale.com>
Acked-by: Liam Girdwood <lrg@ti.com>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
Cc: stable@kernel.org