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/am335x_shc_ict_defconfig b/configs/am335x_shc_ict_defconfig
index 571c9e5..6232440 100644
--- a/configs/am335x_shc_ict_defconfig
+++ b/configs/am335x_shc_ict_defconfig
@@ -9,6 +9,7 @@
 CONFIG_SPL_LIBDISK_SUPPORT=y
 CONFIG_SPL_MMC_SUPPORT=y
 CONFIG_SPL_POWER_SUPPORT=y
+CONFIG_SPL_SERIAL_SUPPORT=y
 CONFIG_SHC_ICT=y
 CONFIG_SERIES=y
 CONFIG_SPL_STACK_R_ADDR=0x82000000