rtc: add config flag to denote rtc API availability

RTC drivers now select RTC, so that code which depends on them
can implement fallback behavior for systems that lack the
hardware or driver.

Change-Id: I0f5a15d643b0c45c511f1151a98e071b4155fb5a
Signed-off-by: Patrick Georgi <pgeorgi@chromium.org>
Reviewed-on: http://review.coreboot.org/9953
Tested-by: build bot (Jenkins)
Reviewed-by: Aaron Durbin <adurbin@chromium.org>
diff --git a/src/Kconfig b/src/Kconfig
index 8c55837..d7fff15 100644
--- a/src/Kconfig
+++ b/src/Kconfig
@@ -345,6 +345,10 @@
 source "src/drivers/*/Kconfig"
+config RTC
+	bool
+	default n
 config TPM
 	default n