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.be


Signed-off-by: Vinod Koul <vkoul@kernel.org>