rtc: make rtc-ds1553 driver hotplug-aware

The rtc-ds1553 platform driver name doesn't match its module name, which
might prevent it from properly hotplugging.  This driver has no in-tree
users.

Signed-off-by: Atsushi Nemoto <anemo@mba.ocn.ne.jp>
Cc: Alessandro Zummo <a.zummo@towertech.it>
Cc: David Brownell <david-b@pacbell.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>