dmaengine: break out channel registration

In preparation for dynamic channel registration, the code segment that
does the channel registration is broken out to its own function.

Signed-off-by: Dave Jiang <dave.jiang@intel.com>
Link: https://lore.kernel.org/r/157965022778.73301.8929944324898985438.stgit@djiang5-desk3.ch.intel.com
Signed-off-by: Vinod Koul <vkoul@kernel.org>