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-socfpga/Kconfig b/arch/arm/mach-socfpga/Kconfig
index 87402e1..1c10ffb 100644
--- a/arch/arm/mach-socfpga/Kconfig
+++ b/arch/arm/mach-socfpga/Kconfig
@@ -15,6 +15,9 @@
 config SPL_NAND_SUPPORT
 	default y if SPL_NAND_DENALI
 
+config SPL_SERIAL_SUPPORT
+	default y
+
 config TARGET_SOCFPGA_ARRIA5
 	bool
 	select TARGET_SOCFPGA_GEN5