sh: add support for SMSC Polaris platform

Polaris is an SMSC reference platform with a SH7709S CPU and LAN9118
ethernet controller.  This patch adds support for it.

Updated following feedback from Nobuhiro Iwamatsu.

Signed-off-by: Steve Glendinning <steve.glendinning@smsc.com>
Reviewed-by: Nobuhiro Iwamatsu <iwamatsu.nobuhiro@renesas.com>
Signed-off-by: Paul Mundt <lethal@linux-sh.org>