ARM: shmobile: koelsch: Rename SCIF[01] serial ports to ttySC[01]

There's no reason to name the only two available serial ports on the
board ttySC6 and ttySC7 (apart from confusing userspace, which we should
try to avoid).

Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Acked-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>