Convert CONFIG_SPL_WATCHDOG_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/ls1043ardb_nand_defconfig b/configs/ls1043ardb_nand_defconfig
index a25d976..a203030 100644
--- a/configs/ls1043ardb_nand_defconfig
+++ b/configs/ls1043ardb_nand_defconfig
@@ -7,6 +7,7 @@
 CONFIG_SPL_SERIAL_SUPPORT=y
 CONFIG_SPL_ENV_SUPPORT=y
 CONFIG_SPL_DRIVERS_MISC_SUPPORT=y
+CONFIG_SPL_WATCHDOG_SUPPORT=y
 CONFIG_DEFAULT_DEVICE_TREE="fsl-ls1043a-rdb"
 CONFIG_FIT=y
 CONFIG_FIT_VERBOSE=y