rtc: Use devm_platform_ioremap_resource()

Simplify probe by using a known wrapper function.

This issue was detected by using the Coccinelle software.

Signed-off-by: Markus Elfring <elfring@users.sourceforge.net>
Link: https://lore.kernel.org/r/4552ef52-f218-93b1-6dfa-668d137676f8@web.de
Link: https://lore.kernel.org/r/5ecfcf43-d6b2-1a38-dee8-b8806f30bc83@web.de
Link: https://lore.kernel.org/r/25448e11-c43f-9ae0-4c43-6f789accc026@web.de


Reviewed-by: Akinobu Mita <akinobu.mita@gmail.com>
Link: https://lore.kernel.org/r/8c17a59c-82ff-aa6b-5653-a38d786d3e83@web.de


Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>