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/db-88f6820-gp_defconfig b/configs/db-88f6820-gp_defconfig
index c001a23..3cdf1b4 100644
--- a/configs/db-88f6820-gp_defconfig
+++ b/configs/db-88f6820-gp_defconfig
@@ -4,6 +4,7 @@
 CONFIG_SPL_LIBGENERIC_SUPPORT=y
 CONFIG_TARGET_DB_88F6820_GP=y
 CONFIG_SPL_I2C_SUPPORT=y
+CONFIG_SPL_SERIAL_SUPPORT=y
 CONFIG_DEFAULT_DEVICE_TREE="armada-388-gp"
 CONFIG_BOOTDELAY=3
 CONFIG_SPL=y