drivers/char/rtc: disable legacy RTC driver on Blackfin systems

Blackfin platforms do not support the hardware which this driver drives.

Signed-off-by: Mike Frysinger <vapier@gentoo.org>
Cc: Christoph Hellwig <hch@lst.de>
Cc: Paul Mundt <lethal@linux-sh.org>
Cc: David Brownell <david-b@pacbell.net>
Cc: Alessandro Zummo <a.zummo@towertech.it>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>