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/spl.c b/arch/mips/mach-mtmips/spl.c
index 95201b8..fe5b49e 100644
--- a/arch/mips/mach-mtmips/spl.c
+++ b/arch/mips/mach-mtmips/spl.c
@@ -17,7 +17,7 @@
 {
 	spl_init();
 
-#ifdef CONFIG_SPL_SERIAL_SUPPORT
+#ifdef CONFIG_SPL_SERIAL
 	/*
 	 * mtmips_spl_serial_init() is useful if debug uart is enabled,
 	 * or DM based serial is not enabled.