rtc: rtc-rs5c313: Convert to module_platform_driver()

Reduce boilerplate by using the module_platform_driver() helper.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Link: https://lore.kernel.org/r/20200814110731.29029-4-geert+renesas@glider.be