rtc: ds1672, max6900, max8998: Add MODULE_DEVICE_TABLE

The device table is required to load modules based on modaliases.
After adding MODULE_DEVICE_TABLE, below entries will be added to
modules.alias:

alias i2c:ds1672 rtc_ds1672
alias i2c:max6900 rtc_max6900
alias platform:lp3974-rtc rtc_max8998
alias platform:max8998-rtc rtc_max8998

Signed-off-by: Axel Lin <axel.lin@gmail.com>
Cc: Alessandro Zummo <a.zummo@towertech.it>
Cc: Dale Farnsworth <dale@farnsworth.org>
Cc: Minkyu Kang <mk7.kang@samsung.com>
Cc: Joonyoung Shim <jy0922.shim@samsung.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>