rtc: rtc-sh: Add support for periodic IRQs.

This adds support for periodic IRQs to the rtc-sh driver.
RTC_IRQP_READ/RTC_IRQP_SET are added, with a number of other fixes and
reordering across the rest of the code.

Signed-off-by: Angelo Castello <angelo.castello@st.com>
Signed-off-by: Giuseppe Cavallaro <peppe.cavallaro@st.com>
Signed-off-by: Paul Mundt <lethal@linux-sh.org>