rtc: m41t62: Convert the RTC driver to support the driver model (DM)

After this change the m41t62.c can be used with RTC subsystem (i.e. date
command) which uses device model (DM).

Signed-off-by: Lukasz Majewski <lukma@denx.de>
Reviewed-by: Stefan Roese <sr@denx.de>