Remove the DT-based Renesas SHDMA DMA multiplexer driver, as it is
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").
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Link: https://lore.kernel.org/r/e9445a5f4ac15fc4d3b376b5e675e39f8c95b967.1623406640.git.geert+renesas@glider.beSigned-off-by: Vinod Koul <vkoul@kernel.org>