dmaengine: rcar-dmac: store channel IRQ in struct rcar_dmac_chan

The IRQ number is needed after probe to be able to add synchronisation
points in other places in the driver when freeing resources and to
implement a device_synchronize() callback. Store the IRQ number in the
struct rcar_dmac_chan so that it can be used later.

Signed-off-by: Niklas Söderlund <niklas.soderlund+renesas@ragnatech.se>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>