rtc: Rename RTC_SUPPORT to RTC

Rename these options so that CONFIG_IS_ENABLED can be used with them.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/configs/chromebook_link64_defconfig b/configs/chromebook_link64_defconfig
index 10d4fd1..147cf68 100644
--- a/configs/chromebook_link64_defconfig
+++ b/configs/chromebook_link64_defconfig
@@ -38,7 +38,7 @@
 CONFIG_SPL_NET_SUPPORT=y
 CONFIG_SPL_PCI=y
 CONFIG_SPL_PCH=y
-CONFIG_SPL_RTC_SUPPORT=y
+CONFIG_SPL_RTC=y
 CONFIG_HUSH_PARSER=y
 CONFIG_CMD_CPU=y
 CONFIG_CMD_GPIO=y