ARM: dts: r8a73a4: Remove non-functional DMA support

The DMA multiplexer node and one DMA controller instance are present,
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/21d4e03c906f5a4a6d9bf5d88d0fc94c069e6325.1623406640.git.geert+renesas@glider.be