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/arch/arm/cpu/armv7/omap4/Kconfig b/arch/arm/cpu/armv7/omap4/Kconfig
index f24be67..c3dc95f 100644
--- a/arch/arm/cpu/armv7/omap4/Kconfig
+++ b/arch/arm/cpu/armv7/omap4/Kconfig
@@ -30,6 +30,9 @@
 config SPL_POWER_SUPPORT
 	default y
 
+config SPL_SERIAL_SUPPORT
+	default y
+
 choice
 	prompt "OMAP4 board select"
 	optional