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>

Documentation/devicetree/bindings/dma/renesas,rcar-dmac.txt

@@ -30,8 +30,9 @@ Required Properties:
 
 - interrupts: interrupt specifiers for the DMAC, one for each entry in
   interrupt-names.
-- interrupt-names: one entry per channel, named "ch%u", where %u is the
-  channel number ranging from zero to the number of channels minus one.
+- interrupt-names: one entry for the error interrupt, named "error", plus one
+  entry per channel, named "ch%u", where %u is the channel number ranging from
+  zero to the number of channels minus one.
 
 - clock-names: "fck" for the functional clock
 - clocks: a list of phandle + clock-specifier pairs, one for each entry