Add a driver for the ST M48T86 / Dallas DS12887 RTC.

This is a platform driver.  The platform device must provide I/O routines to
access the RTC.
Signed-off-by: NAlessandro Zummo <a.zummo@towertech.it>
Cc: Greg KH <greg@kroah.com>
Signed-off-by: NAndrew Morton <akpm@osdl.org>
Signed-off-by: NLinus Torvalds <torvalds@osdl.org>