rtc: add NXP PCF8523 support

Add an RTC driver for PCF8523 chips by NXP Semiconductors.  No support is
currently provided for the alarm and interrupt functions.  Only the time
and date functionality is implemented.

Signed-off-by: Thierry Reding <thierry.reding@avionic-design.de>
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>