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 <[e-mail]>
Cc: Alessandro Zummo <[e-mail]>
Cc: Dale Farnsworth <[e-mail]>
Cc: Minkyu Kang <[e-mail]>
Cc: Joonyoung Shim <[e-mail]>
Signed-off-by: Alexandre Belloni <[e-mail]>

drivers/rtc/rtc-ds1672.c

···
 	{ "ds1672", 0 },
 	{ }
 };
+MODULE_DEVICE_TABLE(i2c, ds1672_id);
 
 static struct i2c_driver ds1672_driver = {
 	.driver = {

drivers/rtc/rtc-max6900.c

···
 	{ "max6900", 0 },
 	{ }
 };
+MODULE_DEVICE_TABLE(i2c, max6900_id);
 
 static struct i2c_driver max6900_driver = {
 	.driver = {

drivers/rtc/rtc-max8998.c

···
 	{ "lp3974-rtc", TYPE_LP3974 },
 	{ }
 };
+MODULE_DEVICE_TABLE(platform, max8998_rtc_id);
 
 static struct platform_driver max8998_rtc_driver = {
 	.driver		= {