dt-bindings: dmaengine: Remove SHDMA Device Tree bindings

Remove the Renesas SHDMA Device Tree bindings, as they are unused.
The DMA multiplexer node and one DMA controller instance were added to
the R-Mobile APE6 .dtsi file, but DMA support was never fully enabled,
cfr. commit a1978861 ("dmaengine: sh: Remove R-Mobile APE6
support").

Note that the mux idea was dropped when implementing support for DMA on
R-Car Gen2, cfr. renesas,rcar-dmac.yaml.
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Link: https://lore.kernel.org/r/ba56b7199fcf3516f202389d2c8f836c9ec51e7a.1623406640.git.geert+renesas@glider.beSigned-off-by: Vinod Koul <vkoul@kernel.org>

Documentation/devicetree/bindings/dma/renesas,shdma.txt

-* SHDMA Device Tree bindings
-
-Sh-/r-mobile and R-Car systems often have multiple identical DMA controller
-instances, capable of serving any of a common set of DMA slave devices, using
-the same configuration. To describe this topology we require all compatible
-SHDMA DT nodes to be placed under a DMA multiplexer node. All such compatible
-DMAC instances have the same number of channels and use the same DMA
-descriptors. Therefore respective DMA DT bindings can also all be placed in the
-multiplexer node. Even if there is only one such DMAC instance on a system, it
-still has to be placed under such a multiplexer node.
-
-* DMA multiplexer
-
-Required properties:
-- compatible:	should be "renesas,shdma-mux"
-- #dma-cells:	should be <1>, see "dmas" property below
-
-Optional properties (currently unused):
-- dma-channels:	number of DMA channels
-- dma-requests:	number of DMA request signals
-
-* DMA controller
-
-Required properties:
-- compatible:	should be of the form "renesas,shdma-<soc>", where <soc> should
-		be replaced with the desired SoC model, e.g.
-		"renesas,shdma-r8a73a4" for the system DMAC on r8a73a4 SoC
-
-Example:
-	dmac: dma-multiplexer@0 {
-		compatible = "renesas,shdma-mux";
-		#dma-cells = <1>;
-		dma-channels = <20>;
-		dma-requests = <256>;
-		#address-cells = <2>;
-		#size-cells = <2>;
-		ranges;
-
-		dma0: dma-controller@e6700020 {
-			compatible = "renesas,shdma-r8a73a4";
-			reg = <0 0xe6700020 0 0x89e0>;
-			interrupt-parent = <&gic>;
-			interrupts = <0 220 4
-					0 200 4
-					0 201 4
-					0 202 4
-					0 203 4
-					0 204 4
-					0 205 4
-					0 206 4
-					0 207 4
-					0 208 4
-					0 209 4
-					0 210 4
-					0 211 4
-					0 212 4
-					0 213 4
-					0 214 4
-					0 215 4
-					0 216 4
-					0 217 4
-					0 218 4
-					0 219 4>;
-			interrupt-names = "error",
-					"ch0", "ch1", "ch2", "ch3",
-					"ch4", "ch5", "ch6", "ch7",
-					"ch8", "ch9", "ch10", "ch11",
-					"ch12", "ch13", "ch14", "ch15",
-					"ch16", "ch17", "ch18", "ch19";
-		};
-	};
-
-* DMA client
-
-Required properties:
-- dmas:		a list of <[DMA multiplexer phandle] [MID/RID value]> pairs,
-		where MID/RID values are fixed handles, specified in the SoC
-		manual
-- dma-names:	a list of DMA channel names, one per "dmas" entry
-
-Example:
-	dmas = <&dmac 0xd1
-		&dmac 0xd2>;
-	dma-names = "tx", "rx";