tty: serial: sh-sci: Hide DMA config question

On most Renesas ARM platforms, the SCIF serial ports can be used with
DMA, so most users will want DMA support to be enabled.

On SuperH platforms, SCI(F) serial ports cannot be used with DMA yet
(see also commit 219fb0c1 ("serial: sh-sci: Remove the platform
data dma slave rx/tx channel IDs")), so users will want it disabled to
reduce kernel size.

Hence follow the above rationale to configure the default, unless
CONFIG_EXPERT is enabled.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>