m68k: Use read_persistent_clock64() consistently

We have two ways of getting the current time from a platform at boot
or during suspend: either using read_persistent_clock() or the rtc
class operation. We never need both, so I'm hiding the
read_persistent_clock variant when the generic RTC is enabled.

Since read_persistent_clock() and mktime() are deprecated because of
the y2038 overflow of time_t, we should use the time64_t based
replacements here.

Finally, the dependency on CONFIG_ARCH_USES_GETTIMEOFFSET looks
completely bogus in this case, so let's remove that. It was
added in commit b13b3f51 ("m68k: fix inclusion of
arch_gettimeoffset for non-MMU 68k classic CPU types") to deal
with arch_gettimeoffset(), which has since been removed from
this file and is unrelated to the RTC functions.

The rtc accessors are only used by classic machines, while
coldfire uses proper RTC drivers, so we can put the old
ifdef back around both functions.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Reviewed-by: Baolin Wang <baolin.wang@linaro.org>
Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>