drivers/rtc/rtc-isl1208.c: add support for the ISL1218

The ISL1218 chip is identical to the ISL1208, except that it has 6
additional user-storage registers.  This patch does not enable access to
those additional registers, but only adds the chip name to the list.

Signed-off-by: Ben Gardner <gardner.ben@gmail.com>
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>