rtc: rtc-sh fixes

- simplifies irq set freq
- ioctl() was duplicating functionalities of rtc-dev core
- corrected initialization sequence
- use platform_driver_probe

Signed-off-by: Alessandro Zummo <a.zummo@towertech.it>
Cc: Angelo Castello <angelo.castello@st.com>
Cc: Giuseppe Cavallaro <peppe.cavallaro@st.com>
Cc: Kay Sievers <kay.sievers@vrfy.org>
Cc: Jamie Lenehan <lenehan@twibble.org>
Signed-off-by: Paul Mundt <lethal@linux-sh.org>