sh: sh-rtc carry interrupt rework

This patch modifies the SuperH RTC driver to only
enable carry interrupts when needed. So by default
no interrupts are enabled with this patch. Without
this patch a suspending system will most likely
wake up by the carry interrupt regardless if the
alarm interrupt has been enabled or not.

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