Convert CONFIG_SPL_SERIAL_SUPPORT to Kconfig

Move this option to Kconfig and tidy up existing uses.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/board/ti/common/Kconfig b/board/ti/common/Kconfig
index 5f22e8c..4980a04 100644
--- a/board/ti/common/Kconfig
+++ b/board/ti/common/Kconfig
@@ -36,3 +36,6 @@
 
 config SPL_POWER_SUPPORT
 	default y
+
+config SPL_SERIAL_SUPPORT
+	default y