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/mach-uniphier/Kconfig b/arch/arm/mach-uniphier/Kconfig
index 7c75348..2e824bb 100644
--- a/arch/arm/mach-uniphier/Kconfig
+++ b/arch/arm/mach-uniphier/Kconfig
@@ -12,6 +12,9 @@
 config SPL_NAND_SUPPORT
 	default y if !ARM64
 
+config SPL_SERIAL_SUPPORT
+	default y
+
 config SYS_CONFIG_NAME
 	default "uniphier"