powerpc: use time64_t in read_persistent_clock

Looking through the remaining users of the deprecated mktime()
function, I found the powerpc rtc handlers, which use it in
place of rtc_tm_to_time64().

To clean this up, I'm changing over the read_persistent_clock()
function to the read_persistent_clock64() variant, and change
all the platform specific handlers along with it.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>

arch/powerpc/platforms/8xx/m8xx_setup.c

@@ -192,9 +192,7 @@ void mpc8xx_get_rtc_time(struct rtc_time *tm)
 
 	/* Get time from the RTC. */
 	data = in_be32(&sys_tmr->sit_rtc);
-	to_tm(data, tm);
-	tm->tm_year -= 1900;
-	tm->tm_mon -= 1;
+	rtc_time64_to_tm(data, tm);
 	immr_unmap(sys_tmr);
 	return;
 }