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/arch/sandbox/include/asm/rtc.h b/arch/sandbox/include/asm/rtc.h
index 025cd6c..bf3ac5e 100644
--- a/arch/sandbox/include/asm/rtc.h
+++ b/arch/sandbox/include/asm/rtc.h
@@ -40,7 +40,7 @@
  * @reg:		Register values
  */
 struct sandbox_i2c_rtc_plat_data {
-#if CONFIG_IS_ENABLED(OF_PLATDATA)
+#if CONFIG_IS_ENABLED(OF_PLATDATA) && IS_ENABLED(CONFIG_RTC_SANDBOX)
 	struct dtd_sandbox_i2c_rtc_emul dtplat;
 #endif
 	long base_time;