rtc: ds1685: Use module_platform_driver

Use module_platform_driver for drivers whose init and exit functions
only register and unregister, respectively.

A simplified version of the Coccinelle semantic patch that performs
this transformation is as follows:

@a@
identifier f, x;
@@
-static f(...) { return platform_driver_register(&x); }

@b depends on a@
identifier e, a.x;
@@
-static e(...) { platform_driver_unregister(&x); }

@c depends on a && b@
identifier a.f;
declarer name module_init;
@@
-module_init(f);

@d depends on a && b && c@
identifier b.e, a.x;
declarer name module_exit;
declarer name module_platform_driver;
@@
-module_exit(e);
+module_platform_driver(x);

Signed-off-by: Vaishali Thakkar <vthakkar1994@gmail.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>