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/cm_t335_defconfig b/configs/cm_t335_defconfig
index 99d66cb..40fe861 100644
--- a/configs/cm_t335_defconfig
+++ b/configs/cm_t335_defconfig
@@ -11,6 +11,7 @@
 CONFIG_SPL_NAND_SUPPORT=y
 CONFIG_SPL_POWER_SUPPORT=y
 CONFIG_SPL_SERIAL_SUPPORT=y
+CONFIG_SPL_WATCHDOG_SUPPORT=y
 CONFIG_VERSION_VARIABLE=y
 CONFIG_SPL=y
 CONFIG_SPL_MTD_SUPPORT=y