dm: rtc: Try to handle the localtime() race

At present the sandbox timer uses localtime() which can jump around twice
a year when daylight-saving time changes.

It would be tricky to make use of gmtime() since we still need to present
the time in local time, as seems to be required by U-Boot's RTC interface.

The problem can only happen once, so use a loop to detect it and try
again. This should be sufficient to detect either a change in the 'second'
value, or a daylight-saving change. We can assume that the latter also
incorporates a 'second' change, so there is no need to loop more than
twice.

Signed-off-by: Simon Glass <sjg@chromium.org>