unicore32: move rtc-puv3.c to drivers/rtc directory

The patch moves rtc driver for PKUnity-v3 SoC from arch/unicore32/kernel/
to drivers/rtc/, with renaming it to rtc-puv3.c.
Also, Kconfig, Makefile, and MAINTAINERS are modified correspondingly.

Signed-off-by: Guan Xuetao <gxt@mprc.pku.edu.cn>
Acked-by: Arnd Bergmann <arnd@arndb.de>