dw_dmac: call .probe after we have a device in place

If we don't yet have the platform device for the driver when it is being loaded
we fail to probe the driver. So instead of calling probe() directly we call
platform_driver_register(). It will call the probe() immediately if we have the
device but also makes the driver to work on platforms where the platform device
is created later.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Acked-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>