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/configs/malta_defconfig b/configs/malta_defconfig
index 5854405..8265dae 100644
--- a/configs/malta_defconfig
+++ b/configs/malta_defconfig
@@ -22,4 +22,5 @@
 CONFIG_ENV_IS_IN_FLASH=y
 CONFIG_MTD_NOR_FLASH=y
 CONFIG_PCI=y
+CONFIG_RTC_MC146818=y
 CONFIG_SYS_NS16550=y