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>

src/Kconfig

@@ -345,6 +345,10 @@ menu "Generic Drivers"
 source "src/drivers/*/Kconfig"
 endmenu
 
+config RTC
+	bool
+	default n
+
 config TPM
 	bool
 	default n

src/drivers/ams/Kconfig

@@ -1,6 +1,7 @@
 config DRIVERS_AS3722_RTC
 	bool "AS3722 RTC support"
 	default n
+	select RTC
 
 config DRIVERS_AS3722_RTC_BUS
 	int "AS3722 RTC bus"

src/drivers/pc80/Kconfig

@@ -20,6 +20,7 @@ config DRIVERS_PS2_KEYBOARD
 config DRIVERS_MC146818
 	bool
 	default y if ARCH_X86
+	select RTC
 
 source src/drivers/pc80/tpm/Kconfig
 

src/drivers/ti/tps65913/Kconfig

@@ -25,6 +25,7 @@ config DRIVERS_TI_TPS65913_RTC
 	bool "TI TPS65913 RTC support"
 	default n
 	select DRIVERS_TI_TPS65913
+	select RTC
 
 config DRIVERS_TI_TPS65913_RTC_BUS
 	int "TI TPS65913 RTC bus"

src/soc/rockchip/rk3288/Kconfig

@@ -32,6 +32,7 @@ config SOC_ROCKCHIP_RK3288
 	select BOOTBLOCK_CONSOLE
 	select UNCOMPRESSED_RAMSTAGE
 	select GENERIC_GPIO_LIB
+	select RTC
 
 if SOC_ROCKCHIP_RK3288