powerpc: time-of-day fixes for 32-bit CHRP systems

This makes 32-bit CHRP systems use the RTAS time-of-day routines if
available.  It fixes a bug in the RTAS time-of-day routines where they
were storing a 64-bit timebase value in an unsigned long by making
those variables u64.  Also, the direct-access time-of-day routines
had the wrong convention for the month and year in the struct rtc_time.

Signed-off-by: Paul Mackerras <paulus@samba.org>