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-tegra/Kconfig b/arch/arm/mach-tegra/Kconfig
index d0cc86e..76909ee 100644
--- a/arch/arm/mach-tegra/Kconfig
+++ b/arch/arm/mach-tegra/Kconfig
@@ -9,6 +9,9 @@
 config SPL_LIBGENERIC_SUPPORT
 	default y
 
+config SPL_SERIAL_SUPPORT
+	default y
+
 config TEGRA_IVC
 	bool "Tegra IVC protocol"
 	help