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/malta64el_defconfig b/configs/malta64el_defconfig
index 992c855..63989ce 100644
--- a/configs/malta64el_defconfig
+++ b/configs/malta64el_defconfig
@@ -24,4 +24,5 @@
 CONFIG_ENV_IS_IN_FLASH=y
 CONFIG_MTD_NOR_FLASH=y
 CONFIG_PCI=y
+CONFIG_RTC_MC146818=y
 CONFIG_SYS_NS16550=y