rtc: Allow use of RTC in SPL and TPL

Add Kconfig options so that the RTC can be used in SPL and TPL. This is
helpful for accessing the contents of CMOS RAM, for example.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/configs/chromebook_link64_defconfig b/configs/chromebook_link64_defconfig
index fe6544b..2c51a2f 100644
--- a/configs/chromebook_link64_defconfig
+++ b/configs/chromebook_link64_defconfig
@@ -69,6 +69,7 @@
 CONFIG_SYS_I2C_INTEL=y
 CONFIG_CROS_EC=y
 CONFIG_CROS_EC_LPC=y
+CONFIG_SPL_DM_RTC=y
 CONFIG_SYS_NS16550=y
 CONFIG_SPI=y
 CONFIG_SPL_TIMER=y