serial: Rename SERIAL_SUPPORT to SERIAL

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

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/arch/arm/mach-imx/mx6/Kconfig b/arch/arm/mach-imx/mx6/Kconfig
index 55de6a9..157217e 100644
--- a/arch/arm/mach-imx/mx6/Kconfig
+++ b/arch/arm/mach-imx/mx6/Kconfig
@@ -309,7 +309,7 @@
 	select SPL_OF_LIBFDT if SPL
 	select SPL_PINCTRL if SPL
 	select SPL_SEPARATE_BSS if SPL
-	select SPL_SERIAL_SUPPORT if SPL
+	select SPL_SERIAL if SPL
 	select SPL_USB_GADGET if SPL
 	select SPL_USB_HOST if SPL
 	select SPL_USB_SDP_SUPPORT if SPL