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/mips/mach-mtmips/Kconfig b/arch/mips/mach-mtmips/Kconfig
index 747988a..9f300a9 100644
--- a/arch/mips/mach-mtmips/Kconfig
+++ b/arch/mips/mach-mtmips/Kconfig
@@ -68,7 +68,7 @@
 	select SPL_OF_CONTROL if SPL_DM
 	select SPL_SIMPLE_BUS if SPL_DM
 	select SPL_DM_SERIAL if SPL_DM
-	select SPL_CLK if SPL_DM && SPL_SERIAL_SUPPORT
+	select SPL_CLK if SPL_DM && SPL_SERIAL
 	select SPL_SYSRESET if SPL_DM
 	select SPL_OF_LIBFDT if SPL_OF_CONTROL
 	help