rtc: Allow RTC_DRV_CMOS to be used on SPARC.

Add Sparc to the Kconfig depends list.

Add __sparc___ to address_sparc = 128 ifdef.

Finally, don't be concerned about 24-hour BCD mode support if the RTC
doesn't have a valid IRQ.  We won't even use the alarm code in this
case and the Sparc RTCs have this limitation.

Signed-off-by: David S. Miller <davem@davemloft.net>