rtc: add support for STK17TA8 chip

This patch adds support for the Simtek STK17TA8 timekeeping chip.

The STK17TA8 is quite similar to the DS1553, but differs in register layout
and in various control bits in the registers.  I chose to make this a new
driver to avoid confusion in the code and to not get lost in #ifdefs.

Signed-off-by: Thomas Hommel <thomas.hommel@gefanuc.com>
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>