sh: sh-sci - fix SH7708 support

This patch makes sure the sci serial port driver compiles for sh7708.
The approach taken is to treat the sh7708 as a subset of sh7706, sh7707,
sh7709. sh7708 is very similar to sh7706, sh7707, sh7709, but only
equipped with a single sci port. The platform data in setup-sh770x.c
already limits the number of serial ports for sh7708 to a single one,
so the non-existing scif ports pointed out in sh-sci.h will remain
unused in case of sh7708.

Signed-off-by: Magnus Damm <damm@igel.co.jp>
Signed-off-by: Paul Mundt <lethal@linux-sh.org>