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-zynq/Kconfig b/arch/arm/mach-zynq/Kconfig
index 5a1f7ca..e948551 100644
--- a/arch/arm/mach-zynq/Kconfig
+++ b/arch/arm/mach-zynq/Kconfig
@@ -15,6 +15,9 @@
 config SPL_MMC_SUPPORT
 	default y if ZYNQ_SDHCI
 
+config SPL_SERIAL_SUPPORT
+	default y
+
 config SYS_BOARD
 	default "zynq"