sandbox: Tidy up RTC options

At present we enable the sandbox RTC driver for all builds. Add a separate
Kconfig option to control this, so that it can be disabled in TPL, where
it is not needed.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/drivers/rtc/Kconfig b/drivers/rtc/Kconfig
index 35b6ed4..fcfda28 100644
--- a/drivers/rtc/Kconfig
+++ b/drivers/rtc/Kconfig
@@ -231,6 +231,24 @@
 	  Enable driver for ST's M41T62 compatible RTC devices (like RV-4162).
 	  It is a serial (I2C) real-time clock (RTC) with alarm.
 
+config RTC_SANDBOX
+	bool "Enable sandbox RTC driver"
+	depends on SANDBOX && DM_RTC
+	default y
+	help
+	  Enable the sandbox RTC driver. This driver connects to the RTC
+	  emulator and is used to test the RTC uclasses and associated code,
+	  as well as the I2C subsystem.
+
+config SPL_RTC_SANDBOX
+	bool "Enable sandbox RTC driver (SPL)"
+	depends on SANDBOX && SPL_DM_RTC
+	default y
+	help
+	  Enable the sandbox RTC driver. This driver connects to the RTC
+	  emulator and is used to test the RTC uclasses and associated code,
+	  as well as the I2C subsystem.
+
 config RTC_STM32
 	bool "Enable STM32 RTC driver"
 	depends on DM_RTC