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/Makefile b/drivers/rtc/Makefile
index 447551e..b6c9029 100644
--- a/drivers/rtc/Makefile
+++ b/drivers/rtc/Makefile
@@ -16,7 +16,7 @@
 obj-$(CONFIG_RTC_DS3232) += ds3232.o
 obj-$(CONFIG_RTC_EMULATION) += emul_rtc.o
 obj-$(CONFIG_RTC_HT1380) += ht1380.o
-obj-$(CONFIG_SANDBOX) += i2c_rtc_emul.o
+obj-$(CONFIG_$(SPL_TPL_)RTC_SANDBOX) += i2c_rtc_emul.o
 obj-$(CONFIG_RTC_ISL1208) += isl1208.o
 obj-$(CONFIG_RTC_M41T62) += m41t62.o
 obj-$(CONFIG_RTC_MC13XXX) += mc13xxx-rtc.o
@@ -35,6 +35,6 @@
 obj-$(CONFIG_RTC_RX8010SJ) += rx8010sj.o
 obj-$(CONFIG_RTC_S35392A) += s35392a.o
 obj-$(CONFIG_RTC_STM32) += stm32_rtc.o
-obj-$(CONFIG_SANDBOX) += sandbox_rtc.o
+obj-$(CONFIG_$(SPL_TPL_)RTC_SANDBOX) += sandbox_rtc.o
 obj-$(CONFIG_RTC_ABX80X) += abx80x.o
 obj-$(CONFIG_RTC_ZYNQMP) += zynqmp_rtc.o