ARM: shmobile: defconfig: Enable DMA for serial ports

DMA for (H)SCIF(A|B) serial ports on R-Car Gen2 and RZ/G1 SoCs is
considered stable, hence enable it by default.

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