serial: sh-sci: suspend/resume wakeup support V2

This patch adds wakeup support to the sh-sci driver. The serial
core deals with all details but defaults to wakeup disabled. So
to make use of this feature enable wakeup in sysfs:

echo enabled > /sys/class/tty/ttySC0/power/wakeup

Signed-off-by: Magnus Damm <damm@opensource.se>
Signed-off-by: Paul Mundt <lethal@linux-sh.org>