sparc64: Check for RTC class device in update_persistent_clock().

As we convert the various by-hand sparc64 RTC drivers to use the
generic RTC framework and drivers, we need to keep the NTP
set_rtc_mmss() support via update_persistent_clock() working.

In the end, after all the RTC device cases are converted, this
local set_rtc_mmss() function will be deleted.

Signed-off-by: David S. Miller <davem@davemloft.net>