arm64: renesas: r8a7795: Add dummy dma-controller nodes

Add dummy nodes for the 3 DMA controllers.
This allows to start describing DMA channels for DMA slaves now.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Magnus Damm <damm+renesas@opensource.se>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>