dmaengine: dw-edma: Defer channel IRQ handling to workqueue

On some SoCs (e.g. R-Car S4) the endpoint-side eDMA raises a single
fixed SPI that is hardwired to CPU0 and covers every read and write
channel. Handling channel events directly in that hard IRQ context
serializes the completion processing of all channels on one CPU:
descriptor recycling and refill, client callbacks (the vchan tasklet
runs on the scheduling CPU) and the doorbell writes all funnel through
CPU0, while the handler additionally spins on each channel's vc.lock.
Especially under heavy multichannel load, this contention becomes a
performance bottleneck.

Keep the hard IRQ handler minimal: clear the status, dispatch channel
events, and defer per-channel processing to work items. A work item per
channel preserves ordering while allowing different channels to run in
parallel on any CPU.

Reviewed-by: Frank Li <Frank.Li@nxp.com>
Signed-off-by: Koichiro Den <den@valinux.co.jp>
Link: https://patch.msgid.link/20260717180639.2643243-10-den@valinux.co.jp
Signed-off-by: Vinod Koul <vkoul@kernel.org>

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