rtc: sh-rtc: Add Single IRQ Support

Add support for single IRQ hardware to the sh-rtc driver.

This is useful for processors with limited interrupt masking
support such as sh7750 and sh7780. With this patch in place we
can add logic to the intc code that merges all RTC vectors into
a single linux interrupt with proper masking/unmasking support.

Specify a single IRQ in the platform data to use this new shared
IRQ feature. Separate Periodic/Carry/Alarm IRQs are still supported.

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