i2c-designware: Support multiple cores using same ISR

Add check to make sure that the core is enabled and has outstanding
interrupts. The activity bit is masked due to the fact that it will
stay active even after the controller has been disabled until the
contoller internal state machines have settled.

Signed-off-by: Dirk Brandewie <dirk.brandewie@gmail.com>
Signed-off-by: Ben Dooks <ben-linux@fluff.org>