ARM: dts: r8a7793: Reference both DMA controllers

R-Car Gen2 have two DMA controllers, which are equivalent. Add
references to both dmac0 and dmac1 so the driver can choose which one to
use.
Signed-off-by: Niklas Söderlund <niklas.soderlund+renesas@ragnatech.se>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>