rtc: m41t80: update sysfs entries export

The driver used an old sysfs entry export.
Update it to use the DEVICE_ATTR_XX macro and remove the unnecessary
CONFIG_RTC_INTF_SYSFS macro.

Signed-off-by: Mylène Josserand <mylene.josserand@free-electrons.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>