timekeeping: Switch HAS_PERSISTENT_CLOCK to ALWAYS_USE_PERSISTENT_CLOCK

Jason pointed out the HAS_PERSISTENT_CLOCK name isn't
quite accurate for the config, as some systems may have
the persistent_clock in some cases, but not always.

So change the config name to the more clear
ALWAYS_USE_PERSISTENT_CLOCK.

Signed-off-by: John Stultz <john.stultz@linaro.org>