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>
diff --git a/drivers/rtc/Kconfig b/drivers/rtc/Kconfig
index f822e13..ce2aabf 100644
--- a/drivers/rtc/Kconfig
+++ b/drivers/rtc/Kconfig
@@ -1051,4 +1051,13 @@
 	  Enable support for the Linux driver side of the Tilera
 	  hypervisor's real-time clock interface.
 
+config RTC_DRV_PUV3
+	tristate "PKUnity v3 RTC support"
+	depends on ARCH_PUV3
+	help
+	  This enables support for the RTC in the PKUnity-v3 SoCs.
+
+	  This drive can also be built as a module. If so, the module
+	  will be called rtc-puv3.
+
 endif # RTC_CLASS