Blackfin: dpmc: punt unnecessary RTC_ISTAT clearing

The RTC ISTAT bits do not affect wakeups, and the RTC driver already
takes care of clearing this MMR when necessary.  So drop the useless
clearing in the core Blackfin power code.

Signed-off-by: Michael Hennerich <michael.hennerich@analog.com>
Signed-off-by: Mike Frysinger <vapier@gentoo.org>