rtc: Set valid date after reset

Some RTC chips tend to set garbage date after reset.
This patch sets the date to 2000-01-01 00:00 immediatelly
after the RTC chip reset is issued using the "date reset"
command.

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Heiko Schocher <hs@denx.de>
Cc: Tom Rini <trini@konsulko.com>