fsldma: allow Freescale Elo DMA driver to be compiled as a module

Modify the Freescale Elo / Elo Plus DMA driver so that it can be compiled as
a module.

The primary change is to stop treating the DMA controller as a bus, and the
DMA channels as devices on the bus.  This is because the Open Firmware (OF)
kernel code does not allow busses to be removed, so although we can call
of_platform_bus_probe() to probe the DMA channels, there is no
of_platform_bus_remove().  Instead, the DMA channels are manually probed,
similar to what fsl_elbc_nand.c does.

Cc: Scott Wood <scottwood@freescale.com>
Acked-by: Li Yang <leoli@freescale.com>
Signed-off-by: Timur Tabi <timur@freescale.com>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>