reboot-mode: must depend on CONFIG_DM_RTC

Reading the boot mode from RTC memory requires a real time clock.
Add the missing Kconfig dependency.

Fixes: c74675bd904b ("reboot-mode: read the boot mode from RTC memory")
Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>
diff --git a/drivers/reboot-mode/Kconfig b/drivers/reboot-mode/Kconfig
index d57baac..bb51893 100644
--- a/drivers/reboot-mode/Kconfig
+++ b/drivers/reboot-mode/Kconfig
@@ -24,6 +24,7 @@
 
 config DM_REBOOT_MODE_RTC
 	bool "Use RTC as reboot mode backend"
+	depends on DM_RTC
 	depends on DM_REBOOT_MODE
 	help
 		Use RTC non volatile memory to control the reboot mode. This will allow users to boot