dmaengine: add support to dynamic register/unregister of channels

With the channel registration routines broken out, now add support code to
allow independent registering and unregistering of channels in a hotplug fashion.

Signed-off-by: Dave Jiang <dave.jiang@intel.com>
Link: https://lore.kernel.org/r/157965023364.73301.7821862091077299040.stgit@djiang5-desk3.ch.intel.com
Signed-off-by: Vinod Koul <vkoul@kernel.org>