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-mv784mp-gp_defconfig b/configs/db-mv784mp-gp_defconfig
index 5c86907..a112b19 100644
--- a/configs/db-mv784mp-gp_defconfig
+++ b/configs/db-mv784mp-gp_defconfig
@@ -4,6 +4,7 @@
 CONFIG_SPL_LIBGENERIC_SUPPORT=y
 CONFIG_TARGET_DB_MV784MP_GP=y
 CONFIG_SPL_I2C_SUPPORT=y
+CONFIG_SPL_SERIAL_SUPPORT=y
 CONFIG_DEFAULT_DEVICE_TREE="armada-xp-gp"
 # CONFIG_SYS_MALLOC_CLEAR_ON_INIT is not set
 CONFIG_BOOTDELAY=3