[POWERPC] Add powerpc get/set_rtc_time interface to new generic rtc class

Add powerpc get/set_rtc_time interface to new generic rtc class. This
abstracts rtc chip specific code from the platform code for rtc-over-i2c
platforms.  Specific RTC chip support is now configured under
Device Drivers -> Real Time Clock. Setting time of day from the RTC
on startup is also configurable.

this time without the potentially platform breaking initcall.

Signed-off-by: Kim Phillips <kim.phillips@freescale.com>
Signed-off-by: Paul Mackerras <paulus@samba.org>