ARM: shmobile: r8a7793: Describe DMA for the serial ports

Add DMA properties to all SCIF, SCIFA, SCIFB, and HSCIF device nodes.

Based on similar work for the r8a7791 by Geert Uytterhoeven.

Signed-off-by: Simon Horman <horms+renesas@verge.net.au>
Acked-by: Geert Uytterhoeven <geert+renesas@glider.be>