rtc: NTP: Add CONFIG_RTC_SYSTOHC_DEVICE for NTP synchronization

Currently, CONFIG_RTC_SYSTOHC uses CONFIG_RTC_HCTOSYS_DEVICE which
is originally used by CONFIG_RTC_HCTOSYS, but this rtc device has
some limiations, for example, it must be battery-backed, be able
to work with irq off and through system suspension, etc.

So add CONFIG_RTC_SYSTOHC_DEVICE used exclusively for CONFIG_RTC_SYSTOHC,
it is more lenient compared to CONFIG_RTC_HCTOSYS_DEVICE, and could
be assigned any available RTC in the system.

Default value is CONFIG_RTC_HCTOSYS_DEVICE which is "rtc0" by default.
After this patch, NTP will sync up "rtc0" by default.

Cc: Paul Bolle <[e-mail]>
Signed-off-by: Xunlei Pang <[e-mail]>
Signed-off-by: Alexandre Belloni <[e-mail]>

drivers/rtc/Kconfig

···
 	  the value read from a specified RTC device. This is useful to avoid
 	  unnecessary fsck runs at boot time, and to network better.
 
-config RTC_SYSTOHC
-	bool "Set the RTC time based on NTP synchronization"
-	default y
-	help
-	  If you say yes here, the system time (wall clock) will be stored
-	  in the RTC specified by RTC_HCTOSYS_DEVICE approximately every 11
-	  minutes if userspace reports synchronized NTP status.
-
 config RTC_HCTOSYS_DEVICE
 	string "RTC used to set the system time"
-	depends on RTC_HCTOSYS = y || RTC_SYSTOHC = y
+	depends on RTC_HCTOSYS
 	default "rtc0"
 	help
 	  The RTC device that will be used to (re)initialize the system
···
 	  be useful to reinitialize system time when resuming from system
 	  sleep states. Do not specify an RTC here unless it stays powered
 	  during all this system's supported sleep states.
+
+config RTC_SYSTOHC
+	bool "Set the RTC time based on NTP synchronization"
+	default y
+	help
+	  If you say yes here, the system time (wall clock) will be stored
+	  in the RTC specified by RTC_HCTOSYS_DEVICE approximately every 11
+	  minutes if userspace reports synchronized NTP status.
+
+config RTC_SYSTOHC_DEVICE
+	string "RTC used to synchronize NTP adjustment"
+	depends on RTC_SYSTOHC
+	default RTC_HCTOSYS_DEVICE if RTC_HCTOSYS
+	default "rtc0"
+	help
+	  The RTC device used for NTP synchronization. The main difference
+	  between RTC_HCTOSYS_DEVICE and RTC_SYSTOHC_DEVICE is that this
+	  one can sleep when setting time, because it runs in the workqueue
+	  context.
 
 config RTC_DEBUG
 	bool "RTC debug support"

drivers/rtc/systohc.c

···
 	else
 		rtc_time64_to_tm(now.tv_sec + 1, &tm);
 
-	rtc = rtc_class_open(CONFIG_RTC_HCTOSYS_DEVICE);
+	rtc = rtc_class_open(CONFIG_RTC_SYSTOHC_DEVICE);
 	if (rtc) {
 		/* rtc_hctosys exclusively uses UTC, so we call set_time here,
 		 * not set_mmss. */