rtc: make rtc-ds1742 driver hotplug-aware

The rtc-ds1742 platform driver name doesn't match its module name,
which might prevents it from properly hotplugging.  There is only two
in-tree user of its driver, which are fixed by this patch too.

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