ASoC: mpc8610: replace of_device with platform_device

'struct of_device' no longer exists, and its functionality has been merged
into platform_device.  Update the MPC8610 HPCD audio drivers (fsl_ssi, fsl_dma,
and mpc8610_hpcd) accordingly.

Also add a #include for slab.h, which is now needed for kmalloc and kfree.

Signed-off-by: Timur Tabi <timur@freescale.com>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>