Kconfig: Convert CONFIG_RTC_MC146818 to Kconfig

Move this option to Kconfig and tidy up the two boards which use it.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/drivers/rtc/Kconfig b/drivers/rtc/Kconfig
index 5436509..97cf7d8 100644
--- a/drivers/rtc/Kconfig
+++ b/drivers/rtc/Kconfig
@@ -68,4 +68,12 @@
 	help
 	  Enable s35392a driver which provides rtc get and set function.
 
+config RTC_MC146818
+	bool "Enable MC146818 driver"
+	help
+	  This is a widely used real-time clock chip originally by Motorola
+	  and now available from NXP. It includes a battery-backed real-time
+	  clock with a wide array of features and 50 bytes of general-purpose,
+	  battery-backed RAM. The driver supports access to the clock and RAM.
+
 endmenu