rtc: CONFIGS_RTC_PL031 must depend on CONFIGS_DM_RTC

Building qemu_arm64_defconfig with CONFIGS_DM_RTC=n and CONFIGS_RTC_PL031=y
leads to a build failure.

Adjust the vexpress64 configuration to avoid circular dependency.

Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>
diff --git a/drivers/rtc/Kconfig b/drivers/rtc/Kconfig
index 7fc53a6..9c2d139 100644
--- a/drivers/rtc/Kconfig
+++ b/drivers/rtc/Kconfig
@@ -203,6 +203,7 @@
 
 config RTC_PL031
 	bool "Enable ARM AMBA PL031 RTC driver"
+	depends on DM_RTC
 	help
 	  The ARM PrimeCell Real Time Clock (PL031) is an optional SoC
 	  peripheral based on the Advanced Microcontroller Bus Architecture