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/configs/ma5d4evk_defconfig b/configs/ma5d4evk_defconfig
index 58d0f74..edbea7f 100644
--- a/configs/ma5d4evk_defconfig
+++ b/configs/ma5d4evk_defconfig
@@ -4,6 +4,7 @@
 CONFIG_SPL_GPIO_SUPPORT=y
 CONFIG_SPL_LIBCOMMON_SUPPORT=y
 CONFIG_SPL_LIBGENERIC_SUPPORT=y
+CONFIG_SPL_SERIAL_SUPPORT=y
 CONFIG_FIT=y
 CONFIG_SYS_EXTRA_OPTIONS="SAMA5D4"
 CONFIG_BOOTDELAY=3