[POWERPC] Create add_rtc() function to enable the RTC CMOS driver

In order to use the RTC CMOS driver, each architecture must register a
platform device for the RTC.

This creates a function to register the platform device based on the RTC
device node and verifies that the RTC port against the hard-coded value
in asm/mc146818rtc.h.

Signed-off-by: Wade Farnsworth <wfarnsworth@mvista.com>
Signed-off-by: Paul Mackerras <paulus@samba.org>