ARM: shmobile: r8a73a4: add a DT node for the DMAC

Add a DT node for the only system DMAC instance on r8a73a4. The RT DMAC
can be added later under the same multiplexer, because they can serve the
same slaves and use the same MID-RID values. Configuration data is
supplied to the driver, using a compatibility match string.

Signed-off-by: Guennadi Liakhovetski <g.liakhovetski+renesas@gmail.com>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>