dt-bindings: rcar-dmac: Document missing error interrupt

The documentation for the "interrupt-names" property only mentions the
per-channel interrupts, while the error interrupt has always been
mandatory, too.

Fixes: 10f5c843 ("dmaengine: rcar-dmac: Add device tree bindings
documentation")
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Niklas Söderlund <niklas.soderlund+renesas@ragnatech.se>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>